This question comes up often. I think(opinion) the limiting factors are, Heater core( low pressure for stock styles up to 1955) and Radiator(same considerations) The Engine won't care much(216 and 235). If you are running a "modern" radiator, and don't use a heater(stock type) then you could run a 15 pound cap no problems. But with some 60 to 70 year old parts I would stay as low as possible. I use a 5 pound cap in the 52 1/2 ton. with the "recirculating" dealer installed heater. No problems for the last 2 years( all weather daily driver). Not the best window defroster action, but no leaks, overheats or other issues.
