Overheating - need help diagnosing

[johnwaynejeep]

My 75 C10 overheats when driving under 20mph. above 20mph, it's fine, even in 115 degree weather on the freeway.

I flushed the radiator, heater core and engine, replaced the water pump and upper/lower radiator hoses as part of regular maintenance and there is no fan clutch. I'm taking it in to replace the thermostat next week because I think the bolts are going to break and will need to be drilled and tapped.

My one concern is the fan. I have a 19" 6-blade flex fan. There is 1.5 inches of space between the fan blades and the radiator shroud. Could this space be the cause of the poor air flow? And how do I fix it? I cannot find a replacement fan big enough to fill this shroud. In fact I can't even find a fan bigger than 18".