How much vacum are you getting , what is the power valve rated at , if the engine vacuum drop below the powervalve rating in gear the PV will open & flood the engine so if you need to be 2" or so bleow the vacuum reading minimum , if you have 12" of vacuum a 6.5 is fine but if you have 7" of vacuum it will not work , I would block the heat crossover but it can cause problems with the choke / cold starts , but will run better otherwise