The early GM diesel based on the Olds V-8 was a 350 aka 5.7 We had enough of 'em in the shop at Burch GMC when I worked there in the early '80s. The 6.2 as mentioned above was totally different and designed from the ground up as a diesel. Detroit did have their fingers in the design. The 6.2 is a much better engine than the Olds which had a nasty habit of dropping valves into pistons if pushed too hard.
We had a customer who bought a new 5.7 GMC diesel pickup. He was a painter and when he brought it back, he told us that it ran fine with an empty bed. But the minute he put a bucket of paint in there, it couldn't get out of it's own way. He was wanting us to pull the diesel out and put a 350 gas engine in it's as a warranty repair. He ended up trading the truck in on a new gas Chevy.
The 6.5 was a punched out 6.2.
