Boooyah, you hit my tender spot with this post! I have vibration similar to yours. Started about 6 years ago. I bought a used (but freshly rebuilt) 454 to swap into my 350 '68 SS. I put engine in and got vibration as you described. I rebuilt engine, had it balanced, still vibrated, but at different rpm range. So i took engine back to machine shop. They re-balanced the assembly, but it STILL had a shake. So I took it back a third time; this time saying that I wanted to be there while they balanced it. They thought I was crazy and that the balance would be perfect. So, we set up the crank and bobweights, flywheel and damper (external balance); put it on the balancer and it shook all over the place! It took over 100g of added material to correct the balance.
Anyway, it STILL did not seem smooth enough for me, so I built a new 454, using a block I had laying around. This time I used Callies forged crank and had it internal balanced (by a different shop). Balance job cost me $1,200 (Mallory metal is expensive). So I put the engine in and it's pretty good, but nowhere near perfect. I ripped out this engine and had balance checked by still another shop, who said that the crank assembly was good, but it shook as soon as they put flywheel on!!!!!!!! So, I'm curious to see how the engine feels after this latest tear-down.
But it's only after I joined this forum that I learned I was using wrong crossmember, so that could be a big part of my problem. Plus I bought new GM motor mounts and will use them instead of poly mounts. Hopefully, I'll get a smooth running car for next Spring................
But your vibration could be something else too (hopefully something simple as mentioned above). I got a fabulous Camaro vibrations document from garymac69
on this forum, who (like me) spent years tracking down a vibration. I converted it to .pdf and you can download it at my server: http://csgbenefits.org/camarovibration.pdf