67 Cuda Cooling

This might be an issue with 'pump design' or to put it another way, "pump not meeting spec." Somwhere on the vast www is an article where someone did experiments to increase flow with "modern" pumps. Part of this had to do with adding a back plate on the rotor do decrease rotor - to - timing cover clearance.

It also occurs, that wear and erosion in this area might be a concern.

I don't like "throwing parts" at a problem, but it might be worthwhile to try a different T stat and try and get the best one you can find.

Last, are you using different pulley sizes?