Just an FYI directed at an earlier post in this thread: retarding the timing causes overall engine temperature to INCREASE; conversely, advancing the timing will DECREASE engine temperature.
I'm thinking that mebbe since the overheating problems occurred after the rebuild, could there be a few clearance issues? Maybe the piston and ring fit is too tight? Bearing clearances? I'm just throwing in a few ideas here, take 'em or leave 'em.
But yeah, a lean engine will absolutely run HOT. That's why there's a mixture control on most personal aircraft. If the pilot's engine is running too hot, he'll richen the mixture. Simple.
Oh, and yes ditch the flexfan. Those are jUNk. You're better off with a clutch fan as it moves more air and is less prone to spontaneous explosions on long trips.