I checked all the plugs, and nothing was loose. I double checked the fuel lines and there were no kinks, and I found no vacuum leaks.

I plugged the electric fuel pump back up by itself, isolating the mechanical pump, and the car started fine and idled fine. Ugh.

Could it be that this Chevy 454 pump works best with a non-vented Chevy pickup gas tank, and thus the return line is not getting enough back pressure from the vented gas cap on the Firebird gas tank, and subsequently pushing too much gas back through the return line?