Reply to "Radiator Choices - Cooling System Info"

Most informative sticky! I purchased my 73, #5936 a little over a year ago and am just now figuring out the mods. Before I get to my overheating problem let me describe the engine setup. Most notably I have the Magnuson Dual Screw SC with a couple of Weber carbs, jacking the HP up to the mid 500's, I suspect. When this was installed in late 80's, I believe the cooling fans were modified. I have two Perma-kool Blower fans and a third sucker fan on the downstream side of the radiator. The Perma-kools run simultaneously and appear to run all the time. The third sucker fan runs off of temp, as best I can tell.
The balance of the cooling system is in original configuration, radiator (stock), piping, hoses, etc. I have a 16lb pressure cap, seems to be in good shape and holds pressure.
Now for the symptoms. I go for a 30 mile drive on a cool day, temp hovers right at 190 by the gauge and stays there. The next day i go for the same drive and it creeps up to the red line, 235-250F, but has not pegged or boiled over. this doesn't matter if i'm stop and go or if i'm cruising at highway speed, although at stop-and-go it does move a little higher.
Based on this thread, I'm thinking Air in the radiator and/or bad thermostat? Would much appreciate input on what steps to take in order to sort this out.
Thanks - Mike