340 overheating problem
If that thing, with that huge radiator, overheats "at speed" I'd look for things like
Are you SURE your temperature "is what it is" IE suspect the gauge.
sticking stat.........try a different hopefully QUALITY brand, not the average O'Really
water pump impeller rusted off shaft. You have a heater? Does it "smoke you out?" Heater hoses should both be too hot to touch with engine warmed up
Does it "puke" coolant? If so, find someone (who still) does chemical "blok checks". This is a chemical that changes color if there's combustion gases in the coolant