A couple more thoughts...
Fan distance to radiator? 1 inch away is ok but the further away you go the less effective your fan is.
Timing? Retarded timing will make an engine run hot and in my observation it will do it in a hurry. Timing too advanced will also cause an engine to run hot but it takes longer.
If your carb is running really lean at idle/low speeds, this can add to overheating, also. But I don't know if you have Rochester carb or what.
Also, the brand of thermostat matters. I would avoid a "Fail-Safe" brand of stat. If your engine overheats
even once, this type has brass levers which lock the thermostat in the
full open position...disabling it completely and forever (unless you remove it and disable those little locks). So then you don't have a thermostat, and thinking you do have one when you don't can cause all sorts of needless confusion. And the one I bought did not have an air bleed hole.
Sounds like from what you've written that you need more air flow at low speeds/idle. Try the electric fan idea. Just keep in mind the amount of amps it needs and the cfm rating. 2000 cfm or so will let you do away with the water pump mounted fan, but it takes 20 amps for one of those. Spal fans are good. Beware of anything boasting 3000 cfm with 6 or 7 amp draw (like this one):
https://www.amazon.com/dp/B01MTLGP1Q/ref=sspa_dk_detail_9?psc=1&pd_rd_i=B01MTLGP1Q 
