for the 6.2, not much, depending on transmission, should be pretty much bolt up, re-route a couple wires for power, and done, if you want to turbo the 6.2 or even drop in a 6.5, you will need to custom make a crossover and down-pipe...that is it. Take a look here, I am almost done with my truck, swapped out the 305, with a 1997 6.5 TD, converted to mechanical injection. I also ditched the 700r4 and installed a SM465, hopefully next year I can find a NV4500.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=31505.msg264531#msg264531for the Cummins, your looking at bell-housing adapters, engine mount adapters, possible other clearance issues with cab and radiator, custom exhaust. I think the engine cross-member needs to be tweaked as well to fit the Cummins oil pan. I'm not real positive on Cummins swap stuff.