I'm pretty sure stock vettes (at least from this era) did not have exhaust manifold gaskets.   If you have a C3 Corvette Central catalog look on the exhaust manifold page.   There's a note in bold on that page: "Remember, your Corvette engine did not originally have gaskets between the heads and exhaust manifolds"  I've gotten this info from other sources as well, and if you'd had seen my car when I bought it, you'd probably be pretty sure that the engine had been completely untouched by the two other previous owners. 

I've never seen a VIRGIN Chevy small/big block with exhaust gaskets...at least not the earlier cars, such as Sharks. My '85, with the stainless steel exhaust manifolds, does, however, have gaskets, so...some of the late Shark cars may have come that way.

I would recommend using a good, quality made header gasket for any exhaust...they just seal much better, especially if you let them soak in water for a few minutes before installing them.

A leak at the manifold could absolutely create a lean mixture scenario, depending on the cam timing/specs, and which cylinder it was leaking from...

wouldn't believe it but now that joel has said it was true, i will. can't understand why they wouldn't put a gasket in. i can see when the engine was built but once fired the heat would affect the mating surfaces and the trueness of the block and the manifold would definately change. doesn't make sense to me.

When the heads and ex. manifolds are new, they are machined true(flat). Once the exhaust is bolted up, and properly torqued, they will stay flat, unless the engine gets overheated for some reason. Once used parts are removed, that's when they are likely to flex, and no longer be flat...thus the gaskets in the O/H kits.
You'd be surprised at how many components in the engine and trans have no gaskets...there are many transmissions that have no gaskets between the pump assembly and the case, and they do not leak. As long as the surfaces are true, clean, and the components properly torqued, they're just not needed.

Thanks for answers, Joel, and everyone else.  Looks like the next thing for me to try is to install manifold gaskets, at least on the RH side.  (The left one has never been off, I'm almost positive).   So could the lean condition be caused from a leak at #8 or #6, with STOCK cam, valve timing, etc.?  I could see it if there was a big cam in there with a lot of overlap, but with stock L-48?  I'm going to try it nonetheless. 

Joel is correct.  OEM did not use exhaust manifold gaskets, but they should always be used when reinstalling them.

I rebuilt the carb in an effort to solve this problem.  I didn't notice a lot of play in the throttle shaft (felt pretty tight to me), but not going to rule it out, either.  Still also could be a warped base plate on the carb as Joel points out.   

So I started testing the compression tonight.  I tested 2, 4, 6, and 8.  They all tested around 175 - 190 psi.  That's cold, all plugs out, WOT, and 5 compression strokes.  (This is my first compression test - does that sound right?)  It took 5 strokes  to get it up to that number on some of the cylinders, but then it topped out.   Am I OK here?  I think it's good that they're all consistent, at least.  I was worried I'd find a cylinder with almost nothing.  I'll check the odd cylinders soon, but I'm not expecting to find anything different over there. 

The method, and results look fine...
I always write down each cylinders reading, so I can compare them. You will hardly ever have all 8 with the same pressures, but they do need to be within a certain percentage, 10-15% of each other, or from highest to lowest.