I am having a new cooling problem with my car. It's a 1968 C Body with stock 383. A few months back the fan hit the radiator, causing it to leak, and the car overheated pretty bad. So I replaced the radiator with an aluminum one from summit. Now the car runs nice and cool in most situations. However, when I start going up a hill, like climbing a grade on the freeway, the temp gauge starts to climb. This is regardless of outside tmperature or vehicle speed, so this happens even when it's cold and I'm driving quickly. I am wondering if maybe I have a slight head gasket leak, and so when combustion pressure increases over idle/cruise, it pressurizes the cooling system and causes overheating.

Thoughts?