[Pyrotechnic]

Depends what you want to spend.

Cummins anything is awesome, but tends to be pricey. 4BT's aren't as plentiful as they used to be so prices are going up. The Cummins replacement program for those bread vans was quite awhile back so most of those have been bought up already. You get what you pay for though. A new 4BT or 6BT will get you 300,000-500,000 useful miles depending on how hard you work it.

For something cheap just to drive around, a rebuilt or take out 6.2 replacement will be your best bet. Just get a good injector pump on it and it should last you just fine. Contrary to popular belief these aren't bad engines. For more power you could step up to a 6.5 or 6.5 Turbo. If you get the early mechanical injection version, these are pretty reliable. There isn't a super huge demand for old GM diesels so your cost will not as as high. You may even be able to find a used motor for cheap, and it will drop right into the truck as is.