Vacuum

Lacking other info, get a copy of the owners manual and the Dodge or Plymouth 1974 factory service manuals.
That diagram above indicates 73 and 74 have the same EGR system. I don't trust multi-year Haynes or similar books for stuff like this.

There's three MTSC's in '73 and '74 worth looking at since they have pictures and explanations. That should help you figure out which ones are on your car.
1973 Chrysler Cleaner Air System Highlights, Master Technician Service Conference (Session 302)
1974 Chrysler Cleaner Air System Movie, Master Technician Service Conference (Session 317)
1974 Chrysler Single Venturi Carburetors Movie, Master Technician Service Conference (Session 322)

For example in '73 they show the venturi controlled EGR on this page, 1973 Venturi Controlled EGR (p.9 Session 302)
Diagram is a little different, and just as important, there are also photos.

OSAC
I know these were used on slant six cars in 74 or 75 because I had one. Your Calf 318 I dont know.
1973 Orifice Spark Control System (p. 11 from Session 302)
The delay can cause a lag in throttle response. :(
In any event, it shows ported vacuum as the source, and mentions the possibility of a temperature controlled valve that also connects to manifold vacuum.
The only differences you'll notice between choosing ported or manifold vacuum for distributor will be at idle, and a just above idle. Using manifold may help bring coolant temperatures down idling in hot traffic. It may also cause the engine to run at higher idle rpm.