Before making any complicated or expensive changes check your radiator cap. Unless the system is overfilled with no room for expansion (not likely on the hot rod with that laid back radiator) or the cooling system is overheating you shouldn't have coolant blowing past the cap. In the old days cars didn't have overflow tanks and as long as the system didn't overheat all was well with the world. If you can't test the cap get a new one or better yet, a known good cap and see if the problem goes away.