wat the differents between the 2 & 1 pierece flywheel?

The bolt pattern. I believe the 1-piece has a smaller bolt circle. Hope this helps.

yes there are differences, ie bolt pattern and not sure if the flywheels had counterweights like the flexplates do for autos.

Yeah, the one-piece rear main seal flywheels (87-up) are externally balanced, whereas the older two-piece flywheels have a neutral balance.