Hello Ford Truck experts. Sorry for the long winded text. I have a 1977 F250 Ranger 2wd, 460,C6, P/S, P/B, A/C, 3:73 differential, 4 row desert cooler radiator, 160 degree thermostat, 7 blade fan w/ thermo clutch, motorcraft 4300 carburator. Live near denver in Colorado. I have had this truck since 1991. I rebuilt the engine soon after I got it. Found a builder out of a 1973 Lincoln with the early style intake manifold, good heads etc. Truck has always ran great, a real powerhouse and pulls what ever I want to. I used to pull a 18' ski boat and jet skis to Lake Powell in August just about ever year in over 100 degree heat A/C on without a problem. I started having heating troubles a few years ago. The radiator had a spot on the top tank that started seeping so I took it to a shop and had it and gone through. At about 110k miles, still having intermittent heat problems. The truck would just run a little hot, usually pulling a load and vapor lock, it has never boiled or steamed on me. After the vapor lock issue having quit on me a dozen times or so I replaced the water pump with a high volume Mildone pump, installed high volume 160 degree thermostat, new thermostatic fan clutch. I insulated the fuel line from the pump to carburator, I even got a very accurate dial thermometer and threaded it into one of the fittings directly on the top of the thermostat housing so I could monitor it. I tested the coolent for carbon monoxide before and current show negative. This January I decided I was done messing around and pulled the engine. All of the pistons were scored from the excessive heat so I just had it completely rebuilt by a trusted engine builder. I get it put back together and it runs perfect except it still wants to run hot, regardless of air temp. At 70 degree evening air temp it will run in the 185 to 190 range. At 90 degrees the other day it tried to vapor lock on me but didn't quite do it, it was running at 205 degrees. My hose hydrant will run 20 gpm. When I had the radiator out, I just stuck the hose in the top opening just to see and it flowed completely at 20 gpm out the bottom, never backwashing out the top one bit? With the high flow water pump and thermostat I could see a significant increase in flow across the radiator. I thought it might be flowing too fast to cool, so I put an original style 160 degree thermostat back in it. I have been a auto mechanic for most of my adult life and worked most every kind of early Ford there is. Love the truck and have had it so long, you just know how it runs and when it's off. This has me utterly bewildered. My next step will be look at the Radiator again, this doesn't seem like it can be the issue. If anyone has dealt with a similar problem or thinks I missed something I could sure use some help.
Thanks for reading.