I voted a 12 valve P pump cummins, I would like to drop it in a 92 or 93 GMT400, so you are period correct on the mechanical injection, as far as the GM erea of mechanical injection, heck, if I am that OCD i would do a VE pump, so it is 92 in a 92! I would find a rear bellhousing plate off of a 4bt from p-chassis bread truck, so I could hook a GM version NV4500 to it, that is mostly OCD than anything, Keeping it all GM rather than getting dodge parts.
I think eventually I would like to have a trio of either 3500s or 3500HDs, all visually identical (probably the 159.5" WB reg. cab Chassis cab, white with grey interior, all 2wd of course) and have one with a p-pump Cummins/GM NV4500, one with a LB7 d-max/ZF6, and the last one a tuned up Optimizer 6500 coupled to a GM NV4500. all a little unique, but all quite functional. oh, and it has to look factory of course, so I might need to kidnap 88GMCtruck to do my wiring!
for a future burb swap, a LB7 coupled to a 6 speed allison would be cool. if that requires too much firewall massaging and or body lifting, I would probably get one of those trick 4L80E 6 speeds from TCI, or whoever makes it.
so basically a fleet of bone stock gloss white 2wd rigs with something nifty under the hood is the dream!