We can't tell how long your Super Swampers will last because we don't know how many miles are on them. Figure 20,000 to 30,000 total miles with very good care and frequent balancing and rotating. Since you're keeping your transfer case and running your used tires, you should be able to save some money for a set of 33s and a 4-inch suspension lift that will let you fit them. When it comes time to replace them, we'd recommend a radial mud tire because it sounds like this F-150 is going to be your only transportation for a while.
High Speed Overheat
I've got an '88 Jeep Grand Wagoneer that's mostly stock with 3-inch springs, 31-inch tires, 3.73 gears, and so on. The transmission is original but the transfer case is freshly rebuilt to original specs.
My problem is that I have had the motor rebuilt twice in 10,000 miles. The original motor, with 129,000 miles, was tired and it overheated on the highway and spark-knocked under load at speeds over 50 mph. So a rebuild seemed in order. The "new" motor was strong, almost fast...well, strong anyway. But after 20 minutes at speeds over 65 mph, the temperature will start to climb into the red. It will also spark-knock terribly under any load/hill over 50 mph. I can't use the A/C at all without overheating on the highway. The odd thing is that I can spend all day in 90-degree heat on the trail, at speeds ranging from zero to 5 mph without the temperature moving up at all.
I do have a winch that blocks a little bit of air into the grille, and I have removed the plastic air dam that I've heard will divert air to the motor. But in either case, this doesn't explain the cool temps at 5 mph off-road where most rigs are going to suffer. I've replaced every hose. The radiator is new. The water pump is new. The fan is stock. The fan clutch is new. I've used 160-, 185-, and 195-degree thermostats and even no thermostat.
The culprit, I've heard, is the timing-it's over-advancing. But I've had countless mechanics adjust the timing but no progress. Help me!
We agree that the toughest test for a cooling system is low vehicle speed and high load use, where there is very little airflow moving though the radiator. Which makes your dilemma that much more frustrating because low-speed operation is the only condition where your engine does not overheat. It seems like you've replaced everything that typically causes overheating, so as you suggested it could be an ignition-related problem. You've had the base timing set at idle, but have you checked to see how much total timing the engine runs when rpm increase and the mechanical advance comes in? Ask your mechanic to check this and verify that you are in the range of 30 degrees of total timing. If you're in the 40-plus degree range, you could be building too much cylinder pressure, which could cause piston-killing detonation and high engine temperatures.
If the timing is in the ballpark, our next step would be to question whether the overheating gremlins might stem from improper engine assembly. The wrong (or improperly installed) head gaskets could limit coolant flow at higher engine speeds. Excess silicone sealer could have plugged up coolant passages, causing a restriction. And there is even a possibility that you are spinning the water pump too fast (wrong pulley), causing turbulence that leads to bubbles and air pockets at high speed.
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