You should be able to easily get 20+ out of it. Sounds like a great swap. Nice to know that I'm not the only whacko that wants to drop a 6.5 into something different. The 6.5 weighs several hundred pounds more than a small block.
As for the trans, the governor will be set lower on the diesel trans. The 'Burb will probably have a K-case based trans - if it is a K-Case, you should look into swapping the trans also.
The diesel will generate more heat than the 305. Personally, if the radiator is in good shape I'd give it a shot first.
DO NOT go to the 5.29 gears! High-revs are death to diesels. With the tires you have, the 4.10's will probably be just about right. You want to cruise at about 1800 rpm for the best mileage.
Space might be your biggest problem. The 6.5 is wide. I had to move/change lots of things to get it to fit. Measure the width of the engine where the injector rails go over the valve covers, then see if you can get that into your engine compartment.
Good luck!
1987 Jeep Grand Wagoneer...new 6.5 in process...diamond block, 18:1's, other goodies...