I'll go along with Dave on the driveshaft balance bit.
Check the runout as well.

Imo, statically balanced tires don't cut it.
You can have an imbalance between the inner and outer sides of the wheel.
Modern spin balancers will see this and that's why different size weights and locations between inner and outer sides of the wheel.

Another one that will drive you crazy - and it used to be common on bias plies - is the tire is out of round.
Even when perfectly balanced you'll still get a vibration at speed.

Don't overlook the engine either.
I'm not real knowledgeable about the SBC, but I understand there are different harmonic dampeners and if you have the wrong one....