There's a truck junkyard in Ucon, ID that has a whole truck with a 4bt and tranny and whatnot in it for something between $4500 and $5000. I was there looking for sundry things and thought to ask about it. They also have a Duramax 6.6 with an Allison on the end of it for $7000. Don't know if the Duramax would fit in your project, but if so it'd be worth the extra $2500. This particular engine came out of a motorhome, so if they kept the ECU and other electronics, it'd probably be detuned on horsepower to provide for longevity. I think Chevy did that with their Duramax-engined fleet trucks. Good luck either way.
I'd love to put a diesel in the ride I'm collecting parts for now, but they're just too proud of 'em at all the places where they're sold. So I think I'm gonna get me a 5.7L diesel and make it a stroked and reamed (0.060 over) ~429 cid small block gas burner. Those engines were crap as diesels, but as gas burners they make for righteous motivation.
Did you ever find a 4BT Cummins? I have two that are in bread vans and will sell one of them. One is a Ford chassis and one is GM. Both have 4 speeds and single rear wheels. Your choice for $2750. Located north of Sacramento,CA.