Dodge has maintained the same 23-spline automatic and 29-spline manual arrangment across the 1st and 2nd (maybe more) generations, I believe. That leaves the bolt pattern, and I know an NV4500 is a bolt-in swap for a 1st gen Getrag, so I assume the automatics might be also. Does that help?
You should ask the guys in the 1st gen forum if you haven't already. What you are doing has been done, I'm sure.
I would be very interested in following along on your Chevy/Cummins conversion. I have a K/V 30 srw 1-ton crewcab that I would really love to put a Cummins and NV4500 or NV5600 in along with the NP205 already there.
Good luck!