I am looking at my newly acquired '67 390 4 speed, and I see just a single diaphragm VAC, routed to the ported vacuum on the carb. My thermostat housing is not correct, as it should have a threaded fitting for a vacuum control valve, I think. Did the '67 4 speeds have a dual diaphragm VAC with one side hooked to that valve in the thermostat housing? Thanks for any info.