I've owned my 78 GMC 3/4 ton PU since '84, and most of its life has been with a 350. I couldnt overheat that engine in the worst conditions (summer heat, towing, AC on, etc). I installed a 454 a couple years back, and had my 27" rad rebuilt as a hi-density 4 row. Now, the engine cooling is only OK, but at freeway speeds, while towing up even a mild grade, the engine runs hot, and forget going up steep grades, it'll overheat. I did all the usual trouble-shooting, rad cleanout, changed thermostats (3 times) fresh coolant, correct timing, fan clutch and shroud, etc, etc. So, my question is, you guys with older trucks and 454's, what size rads are you running, and is this a common problem for older big blocks? Would I be wasting $$ on something like a Ron Davis aluminum rad, or similar? Thanks in advance.