i keep the water level about 3" below the top of the radiator on my TO-35.
i also keep the radiator cap loose,
that way, as the coolant gets warm, it has room to expand without blowing out the overflow tube.
also keeping the radiator cap loose keeps the pressure down on your old radiator.
be sure your coolant is 50/50 solution antifreeze and distilled water.
try this, if it does not work, then you may have a blown head gasket, hope not.
have you done a flow test on the old radiator to be sure its not stopped up, fan belt tight enough to keep water pump turning at the proper rpm.