We had 3 NPRs given to us a few years ago by a mail delivery company. They all had something wrong. Engine, trans, rearend. I don't quite recall the specifics. Anyhow, they just wanted them gone. My old man ended up parting them out. He even found someone wanting the remaining chassis. I think one of them went south of the border. Anyhow, there are deals out there to be had.
From personal experience, I can tell you that nickel and diming a build will take you forever to get anything done. Once you figure out what you want to do, just do it. It'll also help keep you from changing your mind a 100 times.
Anyhow, what are you towing that requires 20,000 lbs of capacity?
Here in the midwest you can also find old grain trucks for cheap at auction. Usually they have 366 or 427 tall decks. I've even seen some with 350s and 292s. They didn't do anything fast, but they got the job done.
