... I managed to back it off and found that the remaining bolt ear was sticking out a tiny amount and was colliding with the camshaft.
This. Or a broken cam or crank key. Or sheared cam gear teeth. An out of time cam means the crank doesn't clear the cam shaft flats, it hits and locks up.
I've never tried it but maybe one way to check is to put it in a higher gear and rock the truck back and forth. Pushing the truck forward the engine will be stuck. But it should turn the engine backward when pushing the truck backward ( in a forward gear).
At least it makes sense in my head.
