Duke, sorry you are having that problem,...non AC radiator would be a 3 core radiator. The AC is a 4 core as far as I know.
I use a 4 core Radiator with extra wide downtubes. No difference in external appearance to original radiator, except it does not have the stamped Harrison on the top.
Go to US Radiator website put in Pontiac and search around. Also call them they are very helpful, and really know the mounts and sizes etc.
That extra cooling from 4 core and larger than stock down tubes adds a bunch of cooling. My 461 runs at 180 has a 160 thermostat.
A couple of little things I drill 3 1/8 holes around thermostat ring to aid the air movement inside, also make sure your fan is half in and half out of the shroud,
and you may want a seven blade Pontiac fan that came on AC cars....much better.
So I would consider upping that Radiator, looks original and that fan as well...that is what the factory did because the condenser dropped all that heat in the engine bay.....let us know how you do.
Oh you will see Jimmy Shine endorsing US Radiator, they are both California shops, he has built a few cars,....and wants performance and original looks....good luck