I have a 454, bored .60 over and a Lunati 1/4" stroker so it is actually a 496. It has a roller cam and Pro Comp aluminum heads with the 2.25 intake valves.
It runs really strong, pulling to 6000 rpm hard. THE PROBLEM is I have a blab at the driver's side tail pipe. I would like to attach a video; but, can't seem
to get it from my cell to send to my computer to download and attach. It only seems to be at idle, as soon as you throttle up it goes away. I put an IR gun on
the exhaust runners and #7 is running 80 to 100 degrees less than the other cylinders. So I attempted a valve lash (they are hydraulic) adjustment, 1/4 turn loose,
thinking the exhaust valve wasn't seating 100%. Made no difference. I am now considering if it is a vacuum intake leak and not pulling the fuel mixture totally.
If anyone else has any thoughts I should check, please respond. I did change out the spark plug, no difference.

 

Burning any oil?
Plug or plugs look oily?
Particularly if #7?
If so that could be a sign of an intake manifold gasket leaking on the inside edge. Block could have been decked, manifold not, so moves up and doesn't seal well. Sometimes leaks oil in back by distrib too.

 

Have you checked with the engine running at night? - plug lead shorting?

 

Carburetor?
It's normal for different cylinders to run at different temps.
FI solved a lot of the problem.
Miss at idle?
I went through this a while ago, and even though the plug wires tested good with my vom, I eventually changed the wires and the miss went away.

 

How many miles are on the engine? I have an C3 with a 427 that has taught me a lot. The solution was to buy a $300 coil according to their techs. They told me that the MSD 6AL was too powerful for the simple coils they offer.
Things like Ignition Coils Fail like that among other things. But then again a spark wire grounding on the frame instead of lighting the mixture via the spark plug would be all it takes to make it misfire.. Several wires started failing which was very noticeable.

The newer spark plug wires by MSD are not lasting for my application. My present problem appeared at 14 months after installing them. I am no longer buying Only MSD brand products. The old MSD wires for my 1968 427 worked awesome until recently, all the sudden the engine is missing and you can hear the spark it is so loud hitting the header pipe or anything within reach. The folding part of the metal inside the MSD spark plug "Boot" allows something to break free and cut the silicone insulation making holes which turn into ground shorts. I can hear them hit the frame the spark is so strong. I Have invested a lot of money in the MSD equipment.

Problems that "Go Away" are the ones that really drive me crazy. If you twisted the wire or anything to help it, okay. I prefer to figure out why, might have something to do with working on piston powered flying machines for fun.

 

I had exactly this issue 'pre' MSD ignition box installation with my 427 and again later when 'ignition box' was installed. Early issue was no doubt related to recent replacement oem style braided HT leads which were total crap.

 

The engine has maybe 500 miles on it. The entire car was a frame up resto mod. The spark plugs looked a little smoked, like it was running rich. No signs of oil, they were dry.
Some of that was the electric choke on the Holley 750 was set too rich and the pull off took awhile. I reset it to lean and it comes off much better.
I just changed the oil and yes between a couple minor leaks and crankcase pressure, it drained out 3 and 3/4 quarts in 500 miles. It was seeping oil at the oil pressure sending unit
and I think a little at the front of the intake. Of course #7 is near the back. All the other exhaust runners were IR temp was 390-410 at a warm idle. #7 was 300 to 325.

 

Change #7 plug and wire.

 

Start with the basics. As RatRacer is suggesting do you have good spark to the number 7 at idle or is it erradic. You could also swap the number 7 plug and plug wire with the number 8 cylinder. Does the miss move? If you have good spark move down.

I would then do a leakdown of that cylinder plus a known good on the other bank, or at the minimum a basic compression test. Do the compression test with the throttle wide open and the adjacent spark plug also out (rules out pressure bleeding over from a blown head gasket) Compare the results. Shouldn't vary more than 10%, but being it's a fresh motor it could be alittle more. A perceived miss can actually be a low contribution situation. You said you checked the lash, perhaps the valve seat wasn't machined properly on one of the valves?

If you have a intake manifold leak (particularly around the runners) you will likely (not always though) see oil on that plug and you will have vacuum in the crankcase whereas normally you have pressure.

This is what I'm immediately thinking of.