I just thought of another item you could try if you went through rohrt's list and still running hot. Try an over flow tank if you don't have one. I don't know when they were first installed in Pontiacs, must be some time around 68?
The radiator is a heat exchanger, transferring some heat from the coolant to the air passing through the core fins, and works best when completely full of coolant fluid. Any air in the radiator will reduce the efficiency of the rad. The cap on a non overflow system holds coolant in the radiator up to the pressure rating of the cap, about 15-16 psi, then releases some fluid to drop the pressure below that which will damage the cooling system. Because the pressure inside the radiator is higher than atmospheric the boiling temperature of the fluid increases to about 245 degrees from the 212 degrees of water at atmospheric pressure. With an overflow recovery system the cap lets the released fluid back into the radiator after it cools thus always having the radiator completely full of coolant and void of air.
Make sure you get a new return style cap with the overflow tank, I run one with a 7 psi rating.

I think the bat wings he's referring to are the X panels.