I hear that the swap to a BBC is pretty slick even with the original trans.
I was going to do the same to an original gas M44 (single rears, chassis only). Bought the engines and the trans's that came with them for the swap - and then sold the truck to someone who wanted to restore it (to tell you the truth, kind of glad that one was going to be restored).
The engines I have are both Tall Deck 427 Truck Engines (I believe the main difference is that the pistons have an extra ring each and maybe a heavier crank than the std BBC). One of my friends had just pulled both out of running trucks, so I bought the second for a spare (brackets, this, that). The first is a fresh rebuilt out of a school bus that comes with an allison auto trans. The second comes with I think a 5-speed manual. I was going to put in the allison so I never checked out the manual. Let me know, I sold the truck and I was just getting motivated to put them on Ebay or Craig's to clear up some space. I think Boyce was one of the guys that used to do the swaps to BBC. Take Care, Dave.