I made/have a diagram explaining how the b body 71/74 AC control switch works and what kind of signal drives every wire. If I can't find where is posted and have patiente will upload it from my laptop in a few hours ( not at home at this moment ). E body coul get diff switch ( dunno ) but wiring color codes is the same.

About using relay for the compressor as mentioned, despite that, blower is the big sucker, not the compressor.You can confirm that just checking how the ammeter flicks with blower on and then just feeding the compressor. Diff is NOTICEABLE. If later models got the relay for the compressor I think is due temp ciclying being controlled by turning on and off the compressor, not due the load. You can use a 4 relays setup for every blower speed ( low, mid, high, and heater which gets its own speed ). I had to made this to save the fan blower lever continuouslly melting the plug on back of my 74 lever selector switch. Have diagrams for that too.

Can't help on vacuum lines thought