Pressure test the radiator/water jacket. You can rent a tester. This will show a blown head gasket or cracked head/cracked block into the water jacket.

Or you can buy the Lisle test kit for exhaust products in the coolant.

Pull the pside valve cover and see if any head bolts are loose (overheat bowed the head and stretched the head bolts to looseness, and the head gasket is gone/going).

If you do the heads, get the radiator rodded. If not rodable, recore with a 3row core for bulletproof cooling, and I betcha the fan clutch is weak, too, but it won't be the cause of high speed overheats - that's the radiator or head gaskets...