Car overheats when going upgrade. Head gasket?

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?

i'D DO A COMPRESSION CHECK ON IT FIRST.

My guess or 2 cents is the cooling system is marginal and when you put a strain on it, it starts to overheats. Maybe that radiator isn't as good as the one you had. Not sure, just throwing it out there.

x 2 on compression check. Also ....see if the cooling system holds pressure also.

Would the compression check indicate that leakage into the water system is occuring? Or prove that maybe I got a few bum cylinders, which require more fuel to produce work, and thus add too much heat to the system?

Do you have bubbles or contaminates in the coolant?

Its without question a radiator problem. The new radiator cant keep up with the demands of the engine.
I would verify ignition timing however...
None of those radiator will cool as well as a factory HD 3-4core unit. Most of the aluminum radiators arent very thick...
How many cores does it have? Whats size is the radiator? How wide are the cores rows?

How is the water pump and thermostat??

Sounds simple, but I would park it on a pretty steep uphill grade. Then check the antifreeze level while running (be careful as usual) and make sure it is full and you don't have an air pocket occuring. Basically make sure the system is "burped".

I "assume" you are using a thermostat.

I say headgasket or cracked head...

It's got a new water pump and new 180° thermostat. I could definitely try to park it on a hill and see if possibly an air bubble. I agree that a factory 4 core is probably better, but the original radiator was the tiny 18" 2 core. Not adequate to start with.

Does it have a thermostatic fan clutch? If so, does the fan clutch engage when it gets warm? Anything showing up in the oil? Any bubbles in the coolant when warm, cap off, looking into the radiator? Is the cap holding pressure? Did you check the thermostat in some hot-to-boiling water before installing it? I've had lazy or just plain "bad" thermostats.

if it is a head gasket leak, you can typically smell exhaust in the coolant. an old mechanic's trick. you'd be amazed at how many times a car comes into our shop after having been all over town, and we have a diagnosis in five seconds. if it isn't the head gasket, it's probably inadequate cooling capacity (radiator).

had the same thing happen on a customers 66 Fury with a 383. Exception being the radiator wasn't replaced. Turns out the block was full of debris from over the years that the block was full up to the bottom of the freeze out plugs!