Could it be as "simple" as stuck valves? E10 loosened years of crud and the valves stuck in their guides? I don't know the GMC V-6s enough to know if there is a valve-piston interference problem, but I'd be thinking stuck valves before a broken timing chain.
An anecdote - I tore down a "runs good" 454 years ago that had shed the nylon portions of the timing sprocket. Even with that, the chain was very loose, but still caught on the remainder of the aluminum sprocket, and hadn't jumped time. Of course the nylon nibs that migrated into the crankshaft oil passages (filter must have clogged and bypassed) didn't do bearing lube any favors. The crank was salvageable, but had to be ground (fortunately it was standard dimension). I know the GMC isn't a big block Chevy, but I'm thinking the GMC is pretty robust.
Tear the engine down, and see what's going on. My two cents
