are you running a radiiator which has no ATF cooler?
the 12 and 24V radiators are very similar - dont need to worry on that at all as long as the cooling system is clean and holds proper coolant.
This picture you show has a bit rust in the radiator, this rust deposit when across the whole engine can reduce cooling capacity by 30 to 40%. Make sure the system is perfecly neat and clean when installing parts - specially cooling system. Real offroad 80 series owners who really depend on them run HD radiators, ELC type coolants and coolant zero charge coolant filters too.
btw - the expansion valve you are using is for R134a, the R12 valves have a different tension in them and slightly lower hysteresis to work correctly in controlling evaporator flooding. The R134a valve should work in R12 - its not end of the world. What I think is happening is you have moisture in system because it wsa vacuumed with a poor compressor that too only to 24 inches of mercury, Which doesnt pull out all moisture. This moisture is becoming icicles on the valve and clogging it - this is amplified if the valve is not wrapped in cork tape when being installed. (ask your mechanic of cork tape - or they called it "kachi rubber")
