Hi Will,
1. First is to determine if your radiator tubes are clogged with grease which usually occurred from over greasing the water pump for years accompanied by never properly cleaning the radiator; or,
2. If your radiator tubes are clogged with mineral deposits from water that contacted the earth like clean well water, or even potable, filtered water from water treatment plants where this water came from lakes, streams or rivers, all of which contain dissolved minerals and can easily build up multiple layers of minerals over time; or,
3. Are your radiator tubes clogged with that of both 1 & 2 above.
4. Grease: The grease is easy to clean with Kitchen dishwater liquid soap or POR 15 Marine Clean, etc., etc., over time; however,
5. Mineral deposits: The mineral deposits require harsher more dangerous chemicals such as caustic soda, (Lye), which one can find under above "Search" ....... harmful to Model A paint, human skin and can cause blindness with careless individuals who never take precaution.
6. Also multiple layers of exterior thick paint on radiators can restrict dissipation of heat or heat transfer ....... if so, use paint remover to remove "all" radiator paint; and paint with very thin, special, "Radiator Paint" from "Professional" auto supply stores.
7. Don't be afraid to look under "Search" and experiment with your radiator ........... Model A folks who sit and complain all of the time about their radiators not functioning properly and overheating are usually those who are too lazy to try "Search" and to try to experiment with something new ........ life always tough for those who don't just stand up and get moving. LOL