Sorry to hear about that.
Hmm...
If it was 'rebuilt' but not run much, then it could have been something not done/done improperly during the 'rebuild'. I had a 4-bolt main 350 one time that was 'rebuilt'. I was installing it in my school bus. For some reason, I decided to put a higher flowing oil pump and cooler on this engine. So when I had it in the stand and flipped it over, I pulled the oil pan. Guess what? the #4 connection rod bolts where just finger snug. That would have ruined my day on the test run! I whipped out the torque wrench and almost everything was loose.
Odd that the piston actually broke apart. Did you do the hydrostatic lock check when you started the truck? Usually the piston will not fail like that unless it hits something or is defective.
If you do pull it apart, we would love to see pictures. Not only so we all can discuss what happened, but we have a gruesome fascination with this kind of stuff.
explain the hydrostatic lock test please
is it cranking the engine with the engine stop lever being pulled to see that the engine cranks freely?
i am really sorry about your engine troubles, good luck