GM Diesels are not very highly rated RPM wise, 3,600 is where the governor is set on most. I've never blown one up and have been unfairly hard on them most of the time I've had a chance to run one. We're talking HMMWV's with 3 speed trans screaming at the P in MPH at the bottom left of the Speedometer for long hauls which should equate to redline and about 80 miles per hour or so. My CUCV is also geared low with a 3 speed and it's just purring at 55 MPH, albeit a little tight. I also run my Multi powered Deuce at around 2600 rpm once it's warmed up. My info likely provides no insight as to why the OP's motor is toast, I just want to point out that these trucks can handle the speed as long as the proper maintenance is done. I am saying, motor failures happen, if you're not prepared to handle a catastrophic motor failure in any of these old trucks, don't mess with them. If you're willing to gamble on them, and judging from bidding prices people have deeper pockets than the new's implicates, have fun with it, get it back on the road and enjoy.