With what he is experiencing I'd bet it's externally balanced.
When I put a stock 400 in a '62 pickup I first hung a 283 flywheel on the end of it. To say that it shook is an understatement. I only drove it around the block and in that short amount of time it wiped out the rear main seal and I was told that there was a great possibility of breaking the crank. I solved the problem with a 454 flywheel.