There could be a lot of different issues that may cause high heat at highway speeds. Do you have a winch or anything blocking air flow through the radiator? Do you have a good fan shroud? I have seen electric fans block air flow at highway speeds. Look at your airflow, is the air going through or around the radiator? A lot of gaps in the front grill, the air will rather go there than through radiator, oil cooler, and auto transmission cooler. Air is going to move from high pressure to low pressure and if you have positive pressure under the hood that electric fan is not going to pull air through the radiator.
I would start with a proper size aluminum radiator, well built fan shroud, mechanical fan with good pitch, fan clutch. And block off all air passage ways so the air can only go through the radiator.
Some High flow water pumps are fine but some are not. Do it right you need to block off the bypass on the passenger head side. Install a total flow bypass thermostat that returns to the water pump. You can find these on the Evans web site. The water will be pumped evenly into both heads and return out the thermostat housing. Then the remote thermostat will return the flow back to the water pump until the temperatures is reached and opens to the radiator. I tried a high flow water pump from Stewart and I blew a heater hose and would not cool.
What pressure radiator cap are you using? And is it working?
There are a few things to get you thinking.
I've been looking at this site for a year now since I bought a '87 Landcruiser and sold one of my early '80s pickup. And now my first post. I may not post much, but there is alot of good infomation here to be learned.
