I have a 1958 and a 1959 Impala. Both have 348's with tri-power and both overheat. I have had the engines rebuilt after one showed combustion gases had entered the coolant. I installed new, 180 degree thermostats, aluminum radiators, Cooling Components electric fans and new water pumps. I run a 50-50 antifreeze mix and have added a can of water wetter. I use 16 lb radiator caps. I have overheating in both city and highway driving. My RPM's at highway speed are in the ranges of 2,300 to 2,500. I am running 32 degrees of mechanical advance. Total advance with the vacuum advance connected is 45 degrees. I have tried running with and without vacuum advance.
I would appreciate any advice as I just don't have any idea of what else to try!
What is your definition of 'overheat'? Are you boiling over? Or just 'high' temps?
If only 'high' temps, what temps are you getting? At idle? At forward speed?
At idle there is little if any mechanical advance; it is 'initial' plus vacuum. Something close to 24 degrees (+,- a few) is what I would call 'typical'. At speed, vacuum advance is nil, timing is 'initial' plus mechanical. If you are 45 degrees BTDC at idle, that looks quite high to me. I don't know what your specific engine needs, nor how it was built. But 45 at idle is way more than GM did.
One thing to try is simply retard your 'initial' timing to around 12 degrees BTDC (with the vacuum advance disconnected and plugged off). Attach the vacuum advance and see if you get up around 24 degrees BTDC, at idle. See if that runs well and reduces your temps. It's free and be be undone easily.
I personally am not a fan (no pun intended) of the electrical fans. A mechanical fan installed properly in its shroud works quite well. It allow full air flow thru the radiator at forward speed and pulls enough air thru the radiator at idle to keep temps within an acceptable range.