High-speed overheating problem

Hey guys. I have a '69 T-bird. The engine temperature is always normal when I'm driving around the city. When I get on the highway, though, and go 80 mph for just 5 minutes, my engine overheats. I replaced the thermostat but that didn't fix the problem. The radiator and water pump work fine (as far as the mechanics can tell). We checked the engine, too. Compression is good on all cylinders. Also, it passed a block test (there isn't any oil in the coolant).

I can't figure this one out. Any ideas?

does you fan work properly, does it have a fan shroud, what kind of tranny does it have.

does you fan work properly, does it have a fan shroud, what kind of tranny does it have.
The fan does work and it has a shroud. It's automatic transmission and there is a bit of slippage. I haven't rebuilt the tranny yet. It is very drivable though

when is the last time the coolant has been changed, and what rpm is the engine at 80mph

I would check that Rad is clean. Make sure no air in system.

Make sure water pump is moving enough water. Could be bad or you may have a buildup in the Rad or the lines.

You may want to add a separate cooler for the tranny fluid.

when is the last time the coolant has been changed, and what rpm is the engine at 80mph
The coolant was changed recently. The car doesn't have a gauge to read RPM! But I know it is very high at 80mph because the car only has 3 gears! If I had to guess, I would say it remains at 4500rpm at 80 mph. Now that I think about it...a lot can go wrong when your RPM is that high for 5 or 10 minutes straight!

I would look into adding separate cooler for the oil and tranny if you are gonna be driving it at that high of rpm for that long.

I would look into adding separate cooler for the oil and tranny if you are gonna be driving it at that high of rpm for that long.
good point. thank you

Put a new radiator and water pump. Make sure that the water is overflowing in the pump. Also ensure that the deflector and the fan molding still on the vehicle and the thermostat placed in the vehicle properly.

Replace the hoses. At high RPM, hoses may be collapsing.