I concur on checking the timing. Are you running manifold or ported vacuum? Running manifold vacuum will cool the engine down. The more vac advance you can pull at idle and part throttle, the cooler the engine will run.

Also, check to make sure you're not collapsing the upper or lower radiator hoses. I put springs in both my upper and lower hoses. The lower spring is usually the culprit, but on Firebirds, they use a very long dogleg upper hose, and sometimes they will collapse (or partially collapse) at the water neck 90 degree bend area due to the weight of the hose when filled.

You can also drill a small 1/8" hole in the thermostat in the outer disc area to allow some bleed through when the car is turned off. This usually eliminates any air pockets from the heater core area.