I've snapped the rear plenum bolt on the left head, and the front plenum bolt on the right head. can I just drill them out and put a bolt in with a nut on the bottom instead of re threading?

 

If you're talking about the intake manifold bolts, I'm not sure about the rear, but I did that with both my fronts as they snapped off due to coolant leaking through the gasket at the front. The left front now leaks coolant down the front of the head so maybe it wasn't such a good idea. I have to take it back apart and try to heli coil it I guess...

The plenum bolts don't go into the head. The plenum is the steel plate on the bottom of the intake manifold that is often replaced with 1/4" aluminum plate to fix the common plenum leak these trucks have.