a flex fan would certainly not hurt you left out some important factors in your post like how many cores your radiator is if its a 3 core you might wanna try swapping far a 4 core try braiding your hoses that helps dissipate heat rather quickly.
i just finished swapping motors in my 73 and had a problem with the motor running hot i put a 17" flex fan on it and a bigger radiator and that solved it. before i was running in the mis 200s to mid 300s degrees when i pu tthat on it dropped to mid 190 to low 200 all the time.
the larger radaitor from years experience when you switch to a larger core you must have a waterpump strong enough to move the extra flow. I've seen some when they made the switch they ran ever hotter & some just stayed the same. Sometimes it's the waterpump not pushing enough flow to begin with. The flex fan will do the trick unless the radiator is already starting to plug (as in old). Just something to think about, have the pressure & flow ckecked with a flow meter. It could save you some $$$$$.
If all else fails, its hard to beat an electric fan. Truck warms up quicker and you control the temp. it runs at, I think some have a manual over-ride also, and they move alot more air than a conventional fan. This probably wouldn't be any more expensive than going to a 4 core radiator conversion.
try running your plow in a different angle or raise or lower it a little to get more air across the radiator. i found that running mine all the way to the top allows more air under the plow and thru the radiator.
I've seen it hundreds of times being in the parts business...
Go out and buy a Thermal Fan Clutch, and a good (high performance) 195 degree thermosdat. Put those two items on and, flush and refil your cooling system with the proper 50/50 mix. Problem solved, I garuntee it.
are you running a snow deflector on the plow, if not get one, I bet that will solve your problem. With out the deflector the air gets pushed over the nose of the truck by the plow. With it, the air is kept in front of the truck and clean air gets over the plow and into the radiator. It will be alot cheaper than new fans and radiators.
It is worth a shot.