If you've checked all the other stuff, especially that the ignition timing isn't retarded causing overheating, It would be worth checking that the radiator hasn't become partially blocked inside after many years of use.
This problem often appears with a rebuilt motor because the extra power the new motor makes pushes more heat into the cooling system.
A good way to check for a partially blocked rad is to slowly bring the revs up on the motor with the rad cap off, you shouldn't see any real change in the water level in the header tank as the motor and water pump speed up if the radiator core is good.
Stuart.
