If I have an internally balanced 383 LT1, which, being an LT1 was originally externally balanced, do I need the ARP external balance or internal balance flexplate bolt kit? Is the difference in the crank (1 piece or 2 piece rear main seal), meaning I still need the external balance bolt set? I will be using the ATI neutral balanced flexplate.

 

The LT1 comes from the factory internal on the front and external on the rear. If your crank is internal on both the front and rear you will need a neutral balanced flexplate. If it is internal only on the front you will need a OEM or replacement flexplate. As far as I know all the bolts are the same.
Note : some cranks do no have chamfers deep enough to accomodate the flywheel bolts, I had to get a 3/4 HSS 82 degree countersink and do my own.