Personally, I would dump the flex fan and electric fan, install a good 7/8 blade fan with HD fan clutch (the ones from the Chevy/GMC 454 trucks work well) and install a good fan shroud.

I live in SW Arizona where cooling is a major consideration. I have a customer with a 70 454 Chevell that was set up almost identical to what you have (except he was using a stock type radiator) and yes he had the same heat problem. When he went to the clutch fan, it cured the heating problem.

I would also stay with a distributor with at least a mechanical advance if your doing any street driving at all with the car.