Having owned a 6.5 I would never recommend the motor to anyone but if you wanted to do it you have some work ahead of you but leave the welder and torch in the shop cause this one is a bolt in deal. the 6.5 weighs in at 728 pounds dry, fully assembled big block is lighter by I wanna say around 100 pounds. Your stock suspension won't really appreciate you doing it but it will take it. The motor mounts will bolt up as they sit, just buy some 6.5 mounts from the parts house. Your best bet as far as a donor motor is going to be a 94 or earlier mechanical injection motor. This is gonna save you alot of work on the wiring side. You can make the mechanical injection engines run with power and fuel but they do still have an ecm that I would recommend having that controls stuff like cold start and A/C idle kicker aswell as lift pump control. It also gets rid of the stupid PMD or FSD, call it what you want. You are going to need to change the fuel tank and the fuel lines. As far as your instrument cluster goes, I don't think you have to change it or the wiring inside the dash but you are going to have to change just about all the wiring under the hood. Best bet on this, get a donor rig or find an untouched 94 with mechanical injection out in the JY and rip it to shreds. Honestly if mileage is what you're after, the 6.5 aint the ticket. In my 3/4 ton extended cab long bed 95 with a 4L80E, 15 was the best I got and it was chipped with injectors and such...on a fresh motor. The DMax is a very enticing swap, there is a guy that put one in his 95 3/4 ton burb and is getting 21 hway but a really big **** eatin grin when he puts his foot down. The 4BT Cummins and 700R4 combo is seeing around 26 but not much power. the 12 valve cummins is the cheaper out the door package but as mentioned before, get ready to do some fabrication. a 10 bolt won't last behind anything but the 4BT cummins and that's because it doesn't produce any more torque than a 350, it just makes it way more efficiently. You need to swap in at least a SF14 Bolt. That's what one guy has in his 88 1/2 ton 4x4 that has a Duramax in. He races it and runs really nice numbers.
The DMax has my vote...that's what I plan on putting in my 93 1/2 ton burb. You will be pleased to know that the front end won't ask anything from you no matter what diesel you go with. I've been reading up alot on the swap over the past few months and have a friend that salvages chevy's so once I get the money you can guess what power plant I'm buyin.