Don't quote me on this but I think the vacuum reservoir was because of the vacuum controls on the AC box inside. My '71 GT has one. The AC/heat controller is all worked off of vacuum not cables, and the blend doors and factory water bypass and all of that work off of vacuum pots. So the reservoir was to run all of that stuff. My '71 is also a smog car, the California '71's were different than the others.
Here's a few shots of the engine compartment as it was when I first got it. The first two were the seller's pictures so don't yell at me about the corrugated radiator hose or goofy water shut off valve and bypassed heater core, that wasn't me! The last picture is mine but the intake was off.
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