'62 Vette

Hello. I've got a restored 62 Vette. It has all new parts. It overheated on me the other day at a cruise. It wasn't very hot out but we were just inching along for about a mile or so when I decided to pull over and it started puking out from the surge tank overflow tube. I know the surge cap is good, it is at least a 12# cap. New Dewitt Alum. radiator, water pump, etc. I know the fan clutch although new, doesn't do it's job, the repops are not good. Any ideas out there?