Yes, tube A used to go to a fume recovery system valve, doubt you have one, it's under full vac all the time on my car/installs....so cap it off, I note that on the right/pass side of the TB, you supposed to have two vac source tubes coming out....one is larger and up top about 7/16 dia, the other is down lower and 3/8 diameter, the smaller one went to a PCV valve.....the larger one went to a valve cover, it's ported vac, coming in before the throttle blades, the PCV valve went on the other valve cover.....
the MAP sensor is a black plastic thing with 3 wires and a weather pac connector on it, and a vac line to main plenum vacuum, usually on one of the ports in the cut out of the rear pass/right side, you fined 2-3? holes in there for screwing in NPT vacuum nipples, one of them is only 1/8 stem and goes to the fuel pressure regulator nice and short hose, the other is 3/16-1/4 hose and goes to the MAP sensor, once again located close by on the firewall on my cars, but I can't recall where GM put it, especialy on an F body which is where your HEI version came from.....I was talking of the HEI like found in a C4 vette in my email.....sorry, I forgot about the F body version with the detached coil.....:nuts:
I saw in one of the pix above a loose wire/connector that looked as if it maybe go to the MAP sensor, hard to tell......MAP sensor is about 3" long with a mounting tab on each end, there maybe variations on that casting though....about 3/8 thick and 1.5" wide.....:bounce:
I suspect that if you hook this all up, and plug that port A, you will be running like a top at ~600 rpm idle, and happy as a clam....