If you run the 8mm that runs through the throttle body back to the top of the header tank then you get constant flow through that which should allow you to use the simple 2 way heater valve & block that circuit off. The Alfa doesn't/didn't have a heater valve so the heater circuit was constant flow. The fail safe way is to use a 4 way heater valve which offers a bypass when the feed to the heater matrix is shut off & retains the constant flow.