Is your new radiator still clean?
Was your block hot tanked?
I had a 383 block that sat for some time with the water galleries exposed to the air & once I started the engine & warmed it up scale started coming out & blocking the tubes in my top tank. I didn't think to check the radiator for some time as I had just re-cored it so I blamed everything else first. I had to get the tanks removed, core rodded re-soldered & I fitted a filter sock on the top hose nipple(& along inside the top tank), which I took off & cleaned every couple of weeks until all the scale had gone. Then I removed the sock & everything was back to normal.
I don't think the "rebuilt" engine I bought was hot tanked!
The scale in the block would not have helped with the cooling either.