Head gasket woes

4700' actual elevation? That must be something like 87kPa barometric pressure? So the engine is really only seeing roughly 87% of sea level load even at WOT. (Farmers math - gets you close!)

So that 36* timing is really not that much timing at that load. It's like only partially opening the throttle on a dyno at sea level.
Hard to understand how both head gadgets would let go at that lighter load. Unless the clamp load just wasn't there.

I don't have the answer but just wanted to ask about the altitude/ point this out.