I'm not sure when 235's started using pressurized cooling systems but at least thru '51, the radiator wasn't pressurized, so a pressure cap really served no purpose. Any of the street rod supply shops can provide an overflow tank, or you can jus use a plastic "jug" from most any mid-50s or later car. I found one from an '80's Malibu that fits the AD fenderwell countour prety well.
