Been done hundreds of times. There's even some selling kits with engine mounts and new cross members that you'll need.
As for the 5spd, I'd go with the nv4500. They were in some of the newer model (like the 88-98) GM trucks and of course behind the Cummins in the 12valve Dodge trucks. IIRC, the only difference is the bell housing and the input shaft. Dodge input shaft was larger and you'd want to go even larger than that if you turn the truck up more than 400rwhp IMO. I can't remember for certain but I'm pretty sure the nv4500 will bolt to an NP205. May need an adaptor but I can't remember for sure.
As for the frame and such in the ol' square body GM, I'm not sure. Like I said, several have done this and I haven't heard of any pretzels. HOWEER, depending on the intended use of this truck, like truck pulling, you may want reinforce the frame cause they aren't nearly as heavy-duty as modern frames. If you don't know, you'll want to DEFINITELY reinforce the frame where the steering pump attaches to the frame and add a support there as well. That part of the frame is prone to cracking regardless. There are several companies that sell the plates and supports to weld in. pirate4x4.com will be your friend.
I HIGHLY recommend you get on there for virtually anything drivetrain and suspension related.
As you'll notice in my sig., I'm doing a duramax in the same vintage truck and am worried about the frame because mine's gonna be a puller. That said, I am considering doing a "body swap" if I can find the right donor truck rather than swapping a duramax et al into the older frame. The modern chassis is just soooo much stronger, I am familiar with them as far as pulling goes and IMO, at the power level I'll be at (2.5 class truck), they transfer power to the ground better/easier.
Back to the Cummins swap though... In searching the web, you'll find there are a couple of "methods" to doing the swap. Some require notching of the frame on the pass. side due to the low mounting of the AC pump on the Dodge/Cummins. There is another way to do it using a bracket that is an oe part for Cummins that mounts the AC pump up higher. There's also a couple of ways to do the engine mounts... IIRC the Box trucks that came with the cummins have a "rake" to them front to back so that they fit. MANY people use these as it lowers the amount of issue with clearance. HOWEVER, I don't think I'd do it that way. I'd run a 4-6" lift up front (it'll compress a considerable amount with the Cummins in there) and have the engine a little lower between the frame rails. This keeps you some trans. tunnel clearance, firewall clearance and the lift keeps the engine off the front diff.
Like I said, if you do some digging, you'll find LOTS and LOTS of info. out there for this. I know because I've considered doing the Cummins in mine because it should be easier and cheaper. I just can't do it though because I wouldn't ever be AS satisified with the truck. Best of luck. I'll look for some good links and post them up....
There's my $0.02
