If you wanted to go cheap on this project, a nice 5 cylinder 81-85 Mercedes Turbodiesel would be the choice. You can pick up an old car fairly cheap and rip out the engine - mating it up might be a bit of a challenge, but those old 3 liter MB engines are pretty good. That's my Plan B... It won't tow as well, but it should rival the stock setup for power - and last forever.
That 4B can be built up for more power if you want. I think you'd be hard pressed to get the torque from the 400 SB that you could from a little Cummins. If you hadn't already repowered with the 400, it would be more cost effective...
