Well, I've spent some time monkeying around with these manifolds.
To answer lionelhutz. Yes they move around a fair bit with all 6 bolts in place. seems one of the 4 outside holes in each manifold is fairly centered. A tapper head bolt as c69vete has suggested sounds like a really good solution. Put in the 2 center bolts. Install the tapper head bolt and snug up. torque up the 5 bolts. remove the tapper head and install the correct bolt and torque.
As much as the pinning idea sounds good. in fact there really isn't much meat to drill in. And drilling straight looks like it could be a real issue.
I know everyone says to run long tube headers to make good power. But I have run headers in the past. Don't like them for a number of reasons. Not to mention this thing made very good power all these years with the 2inch manifolds.
Mostly I am not willing to give up the ignition shielding.
Only Corvettes got this treatment and it is one of the things I won't give up.

So, I will persevere and get these to work.
As per suggestions made by C6_Racer_x. I normally will try to refrain. But in this case I feel I must speak out.
I have been a professional mechanic for almost 50 years now. I actually have taken classes in metallurgy.
I have experienced way to many times in real life the issues with putting stainless steel fasteners in aluminium.
It's called dissimilar metal seizure.
There is a very good reason no manufacturer in the world runs stainless fasteners in Alloy on anything that may need to be disassembled in the future. No there not being cheap.
Way to many times I personally have encountered the Harley owner whom thinks its a great idea to install stainless fasteners in his engine, trans and primary. Why they look great and dont rust! Then 3 or 4 years later the clutch fails and I have to get it apart. And thanks to those stainless fasteners, A 2 hour job becomes a 6 hour job and normally a few more replacement parts. AND you recommend to add loctite as well!
WOW! You never want to get this apart!
I will be running normal stock plated hardware with anti-seize on the threads. these are manifolds not headers. Headers come loose. Manifolds stay put.

 

That is what anti-seize compound is for. Works great and never had a problem.

 

However, you only quoted one small section. The poster not only suggests stainless fasteners, but also high temp loctite on top of it.
Much better suggestion, standard plated fasteners with antiseize.
I STAND MY GROUND.
The properties of stainless fasteners are vastly different than carbon steel. Both shear and tension are vastly different. to simply state that one can torque them up to the same standard is clearly a mistake. Back to, take that course in metallurgy.

 

I wasn't sure with those pictures if it ended up held in one place once all the bolts were in. Flat head screws could be an easy solution to get it in the same location each time.

Most stainless hardware I've seen seems to have cut threads instead of rolled threads. Cranking a bolt in with 1000 little teeth into a threaded hole often doesn't end the best. I might do it on iron heads, but not on anything aluminum if you want the hole to survive.

 

Use studs as a template and fill the oversize holes with high heat JB Weld.

 

I understand the disimilar metals issue but using a SS bolt to go THRU an alum head hole and thread into a cast iron block is not the same problem, is it?

 

Just to update, found a good repeatable method of centering and retaining the retaining the manifolds. Yes the taper head, or flat head screw in the right place with the other misaligned bolts in place did the trick.
I have the manifolds port matched. And the 1404 gaskets arrived, good selection as the port openings on these are large enough as is.
I did note mocking them up that they will need to be glued in place to make certain they stay centered. Best way to go about this?
High temp silicone and glue them to the heads before install?
Any other good ideas?

 

I'd glue gaskets to the manifold vs the head. Easier to remove in the future if needed.

 

putting old manifolds on my gmc van, the bolts did not line up. these manifolds warp some after heating up. then the distance between middle and outer bolts changes. that is the reason for the large outer bolt holes. don't sleeve them, don.t drill guide pins. put all 6 bolts in loose and move manifold around. not enough play to get worked up about. and port matching exhaust manifolds does nothing for power. get gaskets. bolt em on. move on...

 

Scribe vertical and horizontal marks on the heads and manifolds that line up with the port edges where you can see them when assembled. Assemble and see where you need to work. Open up the exhaust manifold ports to match the scribed lines on the heads as required and go a little big on them. Blue tape and a sharpie on machined surfaces will work but not so much on cast surfaces.

I do this with intake manifolds every time. It gives you peace of mind. A bore scope is nice too after the fact.

 

I guess one good thing about this is you wont have to worry about those manifolds cracking due to no room for expansion.

That was a common problem on Pontiac manifolds. The holes were drilled just big enough for the bolts, with no wiggle room. Over time the heat cycles would make them split as the bolts weren't allowing expansion.

 

The porting part is all done now. I understand the expansion issue. Makes sense.
Just looking at ideas for glueing gaskets in place now.

 

I'm going with 2.5" ram horns as well. Picked up mine from Jegs with the black coating. Will need all new exhaust now. I also picked up all new stainless studs. I will have to remember to use the anti-seize if I ever get aluminum heads. They came with aluminum gaskets, but I was planning to use standard gaskets that can be dropped in with the manifold on. Some of you suggesting gluing them in, that sounds like a pain.

 

Yup, it's a tad more work. But when your "Drop in" gaskets block off a portion of your port. I'm thinking this isn't good.
Also, I haven't mentioned where I got my horrible manifolds, but if I were you I would mock yours up. You are going to find the ports are small and do NOT line up.
As per your stainless fasteners, Use antiseize NOW! Your cast iron heads will gall and seize as well. Why do you guys think you know better than the mechanical engineers that design these engines. Stainless fasteners are NOT better.

 

Let's all first understand that the exhaust gases follow the wall. that is even though they are being pushed out. the exhaust gases follow the wall of the pipe or exhaust port.
A close look at muffler design will help explain this. so, if the exhaust leaves the head and their is a slight step down. (Manifold port is slightly larger than the head). Things continue to flow smoothly. however, if the holes in the manifold are smaller than the head, or misaligned there will be a "Step up" . The exhaust following this wall will encounter this step or wall and this will cause it to change direction. this will result in less efficient removal of all exhaust from the cylinder and promote reversion. This is not good.
A port from a Aftermarket 2 1/2 inch manifold

This as delivered. out of the box from a well known U.S. Auto parts house that will remain nameless.
A photo of one of my exhaust ports.

This port has never been touched and although has been run. Has never been modified in any way. This is a Edlebrock head.
What I had to do to remove the uphill steps and insure a smooth flow for my exhaust.

Now I understand that manifolds will never equal headers. But having huge steps and much smaller holes in the manifold will cause these 2 1/2 inch manifolds to perform worse than the factory manifolds.
Glue on my gaskets to make certain they stay centered. You bet. Take my ever loving sweet time on this build to get it right. Damn straight!

 

Glad that trick worked for you. As for gluing the gaskets, even super glue will work for you. No need to overthink it. Once the manifolds and heads get hot the glue will be worthless anyway and probably not cause a problem when you ever have to remove the gaskets.

 

You did the right thing by opening them up......no sense in messing with a 2.5" outlet manifold if the ports cover the ports in the head as much as it seems yours did......
I am doing an "L-46" update right now for a customer and it gets 2.5" 63-65 SHP manifolds with the Corvette Central mandrel bent 2.5" head pipes.......these headpipes are just as important as the 2.5" outlet on the manifold itself......
You lose the scavenging of a long tube with manifolds but you still must make sure they flow........

Jebby

 

Don't forget that even with manifolds it pays to increase the headpipe diameter immediately after the manifold exit, (2-1/2 flared to 2.75" or 3.0"), - if you're making HP or winding up a 327 or larger engine. I'd add an H-pipe, or X-pipe, also at the least.
I built a wicked two into one exhaust with these manifolds yrs ago on our Streetstock racecar (driver's side headpipe was 9-1/2"long into a flowmaster dual 2-1/2" to single 3-1/2"),it had better scavanging than dual pipes with these iron chunks. Randy Brezinski's website has some great info (or used to).

May as well do the best job possible since they do hamper the engine breathing compared to longtubes. $.02
Tim