Tom, that thing is awesome! Completely agree on the underhood temp as the primary problem and it sounds like your experiment will confirm it. Even with the air flow enhancements, you may still have the problem on hot days when idling. If you are using a dropbase air cleaner, it will trap manifold heat and boil the carb. Is there any way to get a thicker spacer under the carb once you open up the fake hood scoop? While I still believe the return regulator would help some, I don't think it is a primary solution. Lowering the underhood temp is though.
By the way, I thought you might like a couple shots of my 1970 Mustang. I put a 460 init and had at least two of the same issues - exhaust (had to fab my own headers and exhaust) and fuel percolation (different air cleaner, cowl induction hood, and return regulator). The cowl induction hood gave me both better air flow in the engine compartment and more room to raise the carb and air cleaner. Just a thought. No problems after those changes:
Mark