I just took my stock radiator to a reputable radiator shop (North Raleigh Radiator) and had them install a core with more tubes and more fins to fit the radiator. They have their own sources for the core. Doing this was the major thing that fixed my "overheating" issues, but I think installing a 190-deg. sleeved thermostat also helped.
If you are restoring the radiator yourself, then the radiator shop can advise you on where to get the core.