Ok here is a list of what i would consider....
PMD known failure due to heat, needs to be moved to a COOL place mounted to a heatsink. Once a PMD is heat damaged it is rolling the dice.
I/P's are often condemed to death before proper diagnostic procedures are completed to see what is actually wrong.
This hummer may be an exception, most hummers came with DB2 injections pumps that are mechanical and not electrical, which means no PMD. But i am not 100% sure!!
Blocks do crack easily, something about the casting that just makes them prone to it, sometimes it happens, most times it doesnt.
Do NOT let the truck get any hotter than 210 degrees F, consider 210 RED. These engine do not like heat like that.
Lift pump needs to be tested, they can go bad and the truck will still run, leading to premature i/p failure.
OPS unit controls if fuel gets to the engine, they run the full voltage of the system however are only rated for less, often replaced unit.
Make sure the starter bracket to the block is still attached to the back of the starter, there is major torque put through it and without the bracked starters shear tabs right off the block..