225,slant six radiator

I believe that you are limited to 19 inches on a 64. 65 and later had a 22 inch radiator support. The larger opening allows use of radiators designed for 70's era A and F bodies. So you can either keep your original radiator, find one spec'd for a 64, or enlarge the opening to the more common 22 inch core opening.

I personally would more worried about the single circuit brake master cylinder than I would be about overheating. It is much harder (+expensive) to repair mangled sheet metal following a brake failure than it is to repair an overheated engine.

And you should take the heater control valve off your heater core hose. You car doesn't use it and it is a potential leak. Looks like someone took a complete engine out of a volare.