Probably a lot cheaper/simpler to put your diesel in the otherwise reasonably original truck.
These things don't have an abundance of room to start with, so wrapping the sheetmetal around a whole newer bigger truck could be a huge chore or a big mess.
Making just what you need fit the old truck without bringing along a lot of extra stuff or unsuitable parts is likely the better choice. That's my approach at least and so far I'm very pleased and have no doubt that using the original frame and suspension is the right choice for me. I'm able to solve each part of the build one at a time and pick and choose the best part for the job considering strength/life, fit, and ease of service. Those aren't choices you'll often have with a body swap, there will be a bunch of compromises.
Grigg
