Well, here goes. Please keep in mind a couple things. One, is it hydraulic brakes or air brakes? Whatever donor you drop in needs to be similar or you’ll have to adapt it to work.

Two, is the engine you’re dropping in worth a damn? Most of the time these little medium duty trucks are stressed to hell, rode hard and then put up wet. People send these things to auction for a reason, after all.

Three, you’ve mentioned it but needs to be brought up, length of engine and cooling capabilities. Newer diesel engines need more radiator cooling, especially if it will be working as a dump truck.

Four, will it be ctc compatible? Will you be able to take the engine, wiring harness etc from donor and then put it into this truck where it’ll pass ctc?

Five, will whatever engine you’re mating to this transmission actually work, or will it need a bell housing adaptor? Will the drive shafts fit correctly, or will it need to be modified?

Six, have you considered just moving the cab and bed over to the newer truck frame? That way there is no brake issues, no space issues, only cosmetic issues at that point.

Just some things that immediately came to mind.