overheat only over 3000 rpm

This could also be an engine running lean. When engines get tired & old, worn rings etc, they lose power. Engine has become less efficient & the engine has to work harder to maintain the same road speed. Since the overheating is happening at cruise speed, it would be worth richening the cruise cct of the carb to see if it helps. Raising the fuel level in the carb by 0.100" will richen the mixture slightly, to see if that helps.

Another method to richen the mixture for a test drive is to close the choke, engine idling, is to close the choke until the engine sound changes. Wire the choke in this position & test drive.