Did this to my brother's "4.3L C10" now a 6.2L C20 about a year ago... here's what i roughly remember
To be swapped:
engine and exhaust (as much as possible - no cats for diesels, and you want true dual exhaust)
radiator (6.2s have a HUGE radiator compared to gassers)
entire fuel system (you can keep your tank and lines, but you need to have a straight pickup (no in-tank pump) or convert the diesel to an electric pump like a 6.5 has)
tranny: can keep the 465 or run the auto, up to you. If you keep the 465 you will need a different flywheel i think. If you go with the auto, you may need new driveshafts
ENTIRE engine wiring harness w/ dual battery setup (good time to install 2 new batteries too)
ENTIRE interior wiring harness(remove stock computer, no computer or anything on an 83 diesel, but glow plug control circuits can be nice)
Instrument cluster (fuel guage and diesel "wait" lights mostly)
fender/tailgate badges
also, you will need to modify the gas filler neck to the larger diesel size (swap or holesaw)
im sure there's stuff im forgetting
