If you want opinions your sure to get a few. I'll lead off with some real world experience.
First check the GVWR and the GCWR of the doner chassis. That means you need to determine the weight of everything...truck, trailer, people, gas, supplies. etc. and if your chassis will handle it then you can start deciding more about the engine.
I like the idea of an E350/E450 cutaway van chassis or a equivelant GM chassis. Both the Powerstroke and the Duramax will handle the job and those chassis should have the weight rating and stopping power. I had a GM cutaway Penske van with a 6 litre Vortec and it would run a lot faster than anyone had any business going with a loaded van so I know the diesel will do it. Great setup with std air, PS, PDB, New bucket seats for your old cab to boot. And you can sell the van body to help cover costs. To me this is a much more practical idea than changing an old truck over to diesel power. You gain all the bonus mechanicals in a proven match. They even already come with the electric brake contoller. It will save tons of time, torment, language, and a LOT of money. Only way to go if you want an old truck look to do a modern day job.
