Overheats when towing after engine swap

[dwmitchell61]

1973 Chevrolet C20 Crew Cab heavy 3/4 ton. Replaced engine & transmission with engine & transmission from 1988 GMC TBI 5.0L and TH400 transmission that shifts good.

Runs great, but...

It overheats when towing a two horse trailer empty going up inclines. Temp gage reads about halfway when it overheats, may not be accurate as 1973 temp gage is connected to 1988 sending unit that was tied in to an electronic (digital) instrument panel.

I checked the OBD-I codes and it only has code 12 stored. Yes, the BATT+ on the TBI system is connected to the truck BATT +, so it should be storing the codes if there wee any.

So I am assuming it is a cooling system issue.

It has a clutched 5 blade fan with single core stock radiator from '70's era. Original radiator in truck developed leaks and was dual core. Original fan was a non-clutched 6 blade flex fan. The radiator cap is 15lb cap. Reutilized water pump from the 1973 5.7L.

New thermostat for 185 degrees was installed with engine swap.
Upper radiator hose replaced with a Gates universal.
New fan belts installed.

Any other suggestions?

1. New radiator cap.
2. Try the flex fan.
3. New fan clutch if flex fan does the trick?
4. New radiator.