Slant Six overheat at cruise speed, especially going uphill

I would bet on the plugged radiator myself but before you do ANYTHING, check to be sure the fan is on the correct way and not backwards. I know, you'll all laugh but you'd be surprised how many times I've seen that. At high RPM on the freeway, the air just won't flow through. The fan is trying to push the air forward at the same time the ram air coming through the grille is trying to flow to the rear.