Faith, when I had my '36 back in the '60's I was running the stock radiator with a small 4 bladed fan and no shroud. The engine was a fairly hot (for the day) 265 with 2 4's. I never ran a pressure cap and the radiator never gave me a problem. I drove the car from Dayton, Ohio to Camp Pendleton and all over So. Cal. Of course the traffic wasn't as bad then as it is now. I guess what I'm trying to say is that I think you might try a regular cap and a catch can for the over flow.