I've had to "lock" the fan clutch in the "on" position on a few of my GM plow trucks--if I didn't the engine would run close to the redline and eventually overheat, and pueke coolant out of the overflow,after some distance with the plow on and blade up in the travel position....
On my '77 GMC with a small block 400,I had tried changing the thermostat,(found none was even in it at first!),and then made sure the radiator wasn't clogged or missing a lot of fins,etc..even tried adjusting the ignition timing around,it didn't seem to make a difference,5-10 miles at 35+ mph would make the gauge eventually peg and it'd peuke coolant into the overflow,or on the ground..
I was expecting the head gaskets to have failed,knowing on 400's they are more prone too,than on the other small blocks--but I pressure tested the radiator and everything held pressure overnight...
It was not until some time later,that a chevy dealer's service tech told me to "lock in the fan clutch" and I'd likely have no more issues--I asked how to do it,he said "All you need is a small screwdriver,might even be able to do it with your fingers--just pull up on the flat "snail" spring where it's tab seats in the slot,and rotate the shaft all the way to the other position,and clip the spring back in again"...
I did that before I left the parking lot,noticed right away the fan sounded like a 747 taking off as I drove away,(as it would normally upon a cold start,but for only a short period,maybe a minute)--you could feel the drag from the fan on the engine and hear it roaring,but on the highway on my way home,I did 60+ mph and the temp gauge never went above 1/2...nor did it ever overheat again...it did suck more gas with the fan clutch "engaged" all the time,so I'd unlock it after spring came..
I've only had to change a few OEM steel shim head gaskets,that died of old age and rotted away in spots enough to leak coolant into the crankcase or blow it into the exhaust..I have only used blue Fel-Pro "Perma-Torque" ones,I'm sure there are many brands just as good,but I've always had good luck with those...