That seems to be a common problem with Pontiacs! The Performance Years BB has a section dedicated to heating and cooling with 90% of it hot motors. Besides the normal stuff, timing, fuel mixture, shroud, there is a lot of discussion about the impeller on water pump, cast preferred over the stamped steel impeller. Another item is the flow of standard thermostats (some guys drill a couple of holes in them). It is either not enough air or not enough water movement (or both).

I just picked up a new water pump for my 67 that I am going to try!