Congrats on the truck!
Of course I am a little diesel obsessed, but I think a 6.5L coupled to a 6 speed Allison out of a Topkick or Kodiak to get the parking brake on the tailshaft would be the cats meow.
If you want to go NV4500, I just happen to have a 96+ (actually out of a 2001 HD) HD spec NV4500 sitting in Idaho. I would trade for a 92-95 HD spec NV4500, as I am one short, and IIRC there was one on Sacremento Craigslist for 750 a while back.
As far as wheels, Cali should be full of them. Alcoa even made rims for them, they look awesome, and they are hard to find, but you stand a good chance of finding them in Cali.
IIRC this bolt pattern goes older, but the bolt pattern and center hole combination has been in use since 1976, most of that time in the P chassis (motorhomes, bread trucks, etc)
Motorhomes stand the best chance of having the Alcoas.
If you cant find any, We have a couple extra sets of steels. I have yet to lay eyes or hands on a set of Alcoas.
If your plan is tow rig, you would be best sticking with your current axles and finding the proper 19.5 rims and staying as a dually. easy to find tires, excellent load capacity, etc.
Like said earlier,
www.3500hd.com is a great resource. there are a few pickup box conversions over there, and they look fine with a dually bed.