In the words of my late father- - - -"Diesels are like sex- - - -highly overrated and stinky!" It would help to know what chassis you're planning to swap into, and what engine you're considering. In general, Diesels are heavy, hard to start in the winter, noisy, and they have a limited effective RPM range so they need a lot of gears.
I have a 120 HP inline 6 Onan/Cummins turbo Diesel that's a little bigger and heavier than a 235 which I'm planning to install in a late 1980's Ford F-350 flatbed. It was used by UPS some years ago, and when Cummins couldn't supply a sufficient number of spares to meet the demand, UPS retrofitted their trucks for another engine. It's somewhat smaller than the Dodge/Cummins engine. This one develops its max HP at 3600 RPM, so I shouldn't have to do any major gear ratio changes.
Earlier designs like the 53 series 2-stroke Detroit engines are horrendously heavy, very noisy because of the screaming blower, and they only have a 400 RPM effective power band (1750-2150). That requires at least 8 and preferably 10 or more forward gears to stay in the power band. Not a good choice, IMHO.
Jerry
