the factory service manual (FSM) for the 1970 Plymouth is available from a few source with googling.
the FSM has some pretty decent charts and diagrams, in color, of the OEM HVAC system on these cars, and could answer many of your questions. I invite you to download a copy, and go through that section. Even if you don't understand it all, you will understand some, and learn, grow, and be a better mechanic.
The only "vacuum canister" I recall, is located and attached directly to the heater box, behind the dash frame assembly.
Section 24 of the FSM is darn informative, and I learn more each time I read through it.