I would get an adjustable timing light. You need to find out exactly how much timing you're running, both mechanical and vacuum. Sounds like your distributor and/or vac advance may be the problem. With an adjustable light, you can determine how much vacuum you have (mechanical) by unplugging the advance (plugging the line), then rev the engine to 3000 rpm and check the total mechanical timing, then do the same with the advance connected. You could have advance weights that are hung up or stuck.

With your combo, you shouldn't have any overheating issues.

I would ditch the electric fan and use a factory clutch fan, or a Hayden aftermarket clutch fan, either the standard duty or heavy duty version. The electric fan is a nice helper (maybe you can mount it front of the radiator), but it's not enough to cool a big Pontiac. Regardless of what the manufacturer claims (they want to sell electric fans), the factory clutch fan, and factory setup, is the best.