Overheating wowes that make no sense - help!!!
If it's overheating at 50 mph, it's not the fans or airflow through the radiator. You don't even need a fan at 50 mph.
Must be a flow (which could include air pockets) or a tuning issue.
Block could be full of crap. Coolant flow speed. Machining.
Yank out the thermostat and test it. Pan of hot water and a thermometer see if or at what temp it opens. That's something free and easy you can start with.