Re: 406 engine question
I don't believe there's such a thing as a 153 tooth "small" fywheel that is externally balanced.
Only two Chevy engines were engineered for external balancing, the SB 400 and the BB 454. On these two mills, there's not enough room in the crank-case for a neutrally balanced stock crank, so weight is added to the flywheel and harmonic to make up the weight that would be on the crank,..if there was room.
From the factory, both these engines got the large, 14" 168 tooth flywheel with biased balancing, though not the same bias; the 400 and 454 FW's are not interchangeable.
In your first post, you say you have an "internally balanced" rotating assy which means everything (flywheel, harmonic, and crank) are neutrally balanced, or should be.
Most aftermarket 400 cranks today are neutrally balanced because it's preferred and because it's easier to balance.
If you combine neutral and biased balanced parts, you'll get some serious engine vibration.
That's why I asked earlier, what brand and model crank is in the engine? That will quickly clear up the issue.