If it bolted up to a stock SM420 bellhousing, it is not an NV4500. Probably either an NV3500 or a T-5. Both of those will bolt up to a stock 420 bellhousing, the NV3500 is a found mostly in 1/2 tons, Jeeps, and compact trucks, the T-5 is basically a passenger car trans, though it also is used in compact trucks as well. The bolt pattern on the NV4500 is very different from the standard GM, and it has a larger index bore ID.
Yes, an adapter plate would mount between the trans and the bellhousing. The difference in your bellhousing is the bolt pattern on the engine side, trans side is the same as standard GM.
