Can you post a picture of the radiator neck with the cap removed, and the attachment point of the overflow tube? A late design radiator with the fill neck designed for a pressure cap will have two gasket surfaces- - - -one above the overflow tube nipple, and another one below it. That design can use a universal fit cap with whatever pressure rating you choose. I'd suggest a 4# cap.
www.ebay.com/itm/186041735906?Jerry
