The 6.5L is a nice engine. It costs a little more to maintain than a gas jobber, and is more difficult to start in the cold, but they get great fuel economy and will move your truck around just fine.
If you run a newer EFI diesel you'll need to add the PCM inside the cab. The ECM is the same size as a TBI version so it can be mounted up behind the glove box. There will be some significant wiring to be done, such as an under-hood fuse & relay block. The electronic 6.5L engines run electric transmissions (4l60e and 4l80e) so you'd want to get the transmission with it. They are mostly the same as gas engines but run a separate torque converter. The ECM will need a 40 tooth tone ring in the transfer case for speed input, which means you'll need to swap an electric speedometer from a 90/91 square body into your truck.
Mechanically its pretty much a bolt in. The turbo will hit your AC box if so equipped and dumps the exhaust directly into the frame rail. I ran my exhaust outside of the frame as there was very little room inside the frame for it. 6.5L engine run very hot, you need to install the big diesel radiator or you'll have over-heating problems. 6.2L rads are the same as the 6.5L stuff. They also have an oil cooler in addition to the power steering and transmission coolers.
You can use a TBI in-tank fuel pump as a lift pump for the diesel. Stock they have a frame mounted pump. Make sure you get as much gas out of the tank as you can. They'll still run with a considerable amount of gas in the fuel but will have issues starting.
Make sure you check the harmonic balancer on the engine when you buy it. If it's swollen, missing pieces or otherwise damaged pass on the engine. When the balancer goes bad then the engine will break the crank shortly afterwards. Keep a close eye on it. Don't worry about cracks in the mains. Pretty much every 6.2 and 6.5L I've dealt with has them. I've never seen or heard of an engine failure attributed to cracked mains.
Make sure you run stanadyne fuel additive. The injection pumps are designed for diesel with sulphur in it. Sulphur adds lubricity to the fuel. The ultra low sulphur diesel we have today will kill the injection pump much, much faster than before. Additive restores that lubricity and keeps them alive as designed.
As mentioned by Rene, you'll want to re-locate the pump drive module. It's on the side of the injection pump stock and they tend to get killed by heat soak. They are supposed to be cooled by the fuel behind the mounting surface but when the fuel stops moving after the engine is shut down they get warmed right up. I mounted mine inside the cab to my seat frame when I ran a 6.5L in the Tahoe. I had no trouble with it there. Most folks mount them to the bumper or something but since I take my truck offroad I was concerned about constantly dunking it in muck and stuff so I put it inside the cab instead.
