I have a 71 Custom Camper 3/4 ton with 350 4bbl AT and factory A/C (which works). It has hot running problems on the highway at speeds over 50 mph when pulling the trailer. God forbid I turn on the A/C. The engine is a crate motor put in in 78 and it also has a newer radiator that was in it when I got the truck that is a 3 row. Did a truck like this come with a 4 row originally? It does have 4 row mounting pads.

how hot is hot?

Hot to pike the gauge with several sending units.

have you confirmed the temp with a secondary method? like a hand held IR sensor? I ask because I've had customers chase an overheating problem when it was a sender, wiring, or gauge problem. Better to eliminate them first thing. with the temp gauge pegged it should be blowing coolant out the overflow

a 3 row with a high fin count will cool as well as a 4 row with a low fin count

what type of fan are you running? some fan designs can impede air flow thru the radiator at highway speeds

I have a 1/2 ton, '71 GMC with 350 V8, std. 3 speed trans, and factory air. It has a very thick radiator, I don't know how many rows of tubes, but it's massive compared to non air trucks of the same age.

Mike

I fixed the hot problem! It was the radiator being too small. It made the water go too fast through the core and not let it cool enough. A new 4 row Heavy Duty radiator and it was smooth sailing with no vapor locking, no gauge spiking over 50mph. I could even run the A/C while towing the trailer (with the 23 Olds wooden cab truck on it, it's like pulling a parachute and a crate full of anvils), on a hot 90 deg afternoon and never got past halfway while idling at a long construction site flagman area. Most driving and it was down towards the low temp area. Happy days!!!!

there is no greater satisfaction than the satisfaction of a job well done

The water can't go thru too fast, The problem not enough water was going thru. Its actually going thru faster now because there is less restriction.....but 25% more is going thru.