Finally - 56 and my first Vette - an 81 C3. Anyway - to start the upgrades, what's the best way to upgrade the exhaust system (from headers back). Are there dual hump crossmembers that can be swapped in, like from earlier models, for real duals, or are there any outfits that make a dual setup following the original single routing that are mandrel bent, OR even a large diameter single setup (which Dynomax says is good for low-mid range torque)?
Eventual plan is for 383 (target is an LT1) with AFR 190 heads, swap to
hp 4 speed auto OD, mild stall and 3:54's, hopefully port efi.

Starting with exhaust, so thanks for any info!

 

You have the right idea-start with an exhaust that will handle your future upgrades. I bought an aftermarket tubular crossmember from Bowtie Overdrives. I still have the double hole crossmember that I took out, but there is one person in line to get it. If the deal falls through with him, it will go on the part for sale section. Personally, I'd go ahead and put on headers and dual exhaust, so if you decide to go to headers in the future, you won't have to have anything redone. Mid America Direct makes a dual bolt up system. Stainless Works also makes a bolt in dual system, and theirs includes mufflers. The downfall to both of these systems is that they both bolt to the original exhaust manifolds.

 

I did what you're thinking about.
383 - small primary headers with 4 primaries into a flowmaster 4-2-1 collector. connect to 2.5" mandrel bent tubes that meet in an X just behind the trans. Continue back with mandrel bends into hiflo muffs. I use Edelbrock RPMs. Use the OE stainless square tips but open up the inlet to 2.5"
This system flows great and isn't too loud. It does reduce ground clearance. I put skid pads on the bottom of the lowest tubes by welding on half pipes below the diff.

 

I have an '81 and I installed the Mid america dual system straight to my existing headers. The system is stainless and doesn't need any modification to the cross member. I am very impressed with the system and was really easy to install. I can highly recommend it to anyone.

 

I'm not sure what kind of crossmember an 81 has, but in my opinion, you might be better off with the tubular crossmember mentioned earlier, vs an older c3 x-member (just in case you get an even more radical idea in the future)
A couple of years ago, I spoke to a mid-america techie, and he gave me a part # that isn't listed in the catalog... off road use only - its an aluminized 2.5" dual exhaust setup that comes with some flanges that bolt right up to the headers, no mufflers included, though. Its the same price as their aluminized dual set that works with the stock manifolds.

In my experience, with an auto trans its kinda tight, but you can still hang her up at home yourself, but you'll need to have it welded. Again, I'm not sure about the '81 x-member, so I'm not sure if this kit will work with what you have right now. Of course, you also have to contend with the smog patroll, and make sure that you can get away with what you're planning, or keep the old stuff on hand and swap for the emission tests. I tweaked my car a little lean, and was able to pass without cats on the idle sniff test, but I wouldn't pass if they dyno'd her on the roller.

I'm interested in getting the q-tech exhaust valves that you can un-cap the exhaust with a turn of a dial. Unfortunately, its kinda tight next to the tranny, and I'll hafta figure out where to place some cutouts where they wont interfere with the shift linkage or modulator. I see now that I'm limited by my stock x-member, and may need to re-route the whole exhaust to be able to run 3" pipe back from the headers.