I am the one doing the 4.3L Diesel swap into a 90 Trooper. I did a lot of research before I attempted this and it is a very doable swap. I also have a 5.7L Diesel and it would not fit, so the 6.2/6.5L Diesel is a no-go.
There are some advantages with the 4.3L, 170ftlbs of torque in stock form is the big plus. Second is that it bolts up to the stock Isuzu trans, if it was a 2.8L equiped truck to begin with. So, no trans swap or adapters needed.
The size of this engine is large, but it still fits in the engine bay. A V8 would not fit.
I am a little worried about the power it will have. I read that people with the 4J**-TC engine have clutch issues due to the "High" power of those engines. Those only have about 120ftlbs of torque and 85hp. Mine has 170ftlbs of torque and it has it low in the RPM range due to being a non-turbo engine. <img src="/forums/images/graemlins/evil.gif" alt="" /> I may need to look for a high pressure pressure plate and heavy duty clutch if this one goes out too soon.
I am having Digital camera problems at the moment so I have no pictures, but I will get some soon.
The most difficult part of this swap is finding a GM FWD car with the 4.3L diesel in it. I got lucky and found one at a local used car dealer and traded an old Geo Metro for it. They do come on E-bay and other sites sometimes.
Good luck and if you have any questions, feel free to ask me.