Double-check your ignition timing (base, vacuum advance, and mechanical advance), since retarded timing will cause high-speed overheating. You will probably also want to double-check your camshaft timing for the same reason. You will also want to check for exhaust restrictions. Have you got a vacuum gauge?
Assuming no timing problems or exhaust restriction, high-speed overheating generally means a water flow problem. Sounds like you've definitely got enough radiator, so focus on other water flow problems. Sometimes you can get a problematic water pump with an eroded or slipping impeller. Have you pulled the pump to inspect it and replace if necessary?
Also, it is probably time to remove the freeze plugs and scrape the mud and compacted rust out of the engine's water jackets, flushing it out of the block via the front (now conveniently open to the world because you removed the water pump!
, install new freeze plugs, and then clean the engine with a strong de-rusting cleaner. Then install a new temperature sender and a Stant SuperStat 45358 thermostat, make sure your radiator pressure cap is in good condition......and if you're still overheating, then it's time to ponder the ugly possibility of a crack in the head. You mention blowoff of steam; you may want to make use of a Block-Chek kit or put an exhaust emissions "sniffer"at the open radiator neck with the engine at a fast idle, to see if there's a combustion leak into the cooling system.