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I agree with Eagle on his question of is it really boiling over or is it spewing water because it is over full.

How full do you have the radiator?
How hot is it?
Have you actually tested the temperature of the water?
Go down to your grocery store or a Bed Bath and Beyond or something like it and get a cheap meat thermometor and actually test the water temperature.

Once you establish that is actually overheating then you can start looking at options to fix it.

Lots of things can cause overheating.
Rust on top of the tubes like Bill says.
Retarded timing.
Fins detached from the tubes.
Paint too thick on the fins.
Fan belt slipping.
Radiator tubes clogged.

Have you checked the torque on the head to make sure it is not pushing combustion gases into the cooling system?

Others will have more ideas I'm sure.