[PANAVIA]

There are two different vacuum sources that kick in at two different time(s) I call them manifold and the other carb. - in the case of old engines , part throttle cruising at 2000-2500 RPM gave high carb vacuum "pull" which had your engine timing advance in that "Cruise" mode - it would add say 5-7 degrees in that situation.

Manifold vacuum was used on early 327 stingrays to give a nicer idle ,adding 5-7 degrees at idle to 2000 RPM then dropping off. --example dropping out at WOT to run static and mechanical advance timing when you were "on it-"

Starting in 1968 - ford had a vacuum switch in the thermostat housing - that had both ported ( carb ) and manifold vacuum sources and depending on temperature it would either apply carb vacuum (COLD) or manifold ( HOT ) to advance timing to have the engine run cooler in traffic with the AC on....

now if you then take smog into mind, they then started applying manifold vacuum on the Dual ported vacuum advance canisters to either advance timing, or even retard timing -= lowering NOX emissions ( again 1960s smog systems - ewww. )

I would try both and see what works best -- keep an eye on operating temperature, - gas consumption , pinging , - and driveability.