The vent is removed by 2 1/4 20 nuts from underneath, Go up through the holes in the underside of the vent assembly with a 7/16 deep well socket and an extension and remove those nuts and the vent will come off.
The AC assembly is removed by the 1/4 20 nuts on the firewall under the hood.There is some behind the engine but you can get to them . The fan should come off first removing the screws facing in on the rim of the fan then the fan slips off the box rim.
There is also a rod inside the passenger vent door that holds the box up to the cowl seal. That is secured with one 1/4" hex screw on the bottom of the box pre 73 and inside the door on 73 - 76 Then its just the wires ,cables, and hoses.. You will have to remove the glove box , ash tray, radio and kick panel to get it down and out easily. Good luck.