What degree Thermostat?

180° is factory equipment and best choice for slant-6s through '73 (some years had 185° or 190°), then they went to 195° through the end of production. Best pick depends on your underhood configuration, ambient temperatures and vehicle usage patterns, the fuel you have available locally, and the kind of coolant you're running. Colder than a 180° stat, or no thermostat, is not a wise choice.

Best thermostat is a Stant SuperStat, 45358 for 180° or 45359 for 195°.

Premium radiator? Sure, see here.

Reading between the lines, it sounds like you might be trying to chase down an overheating problem…?