On my 66 same firewall if factory Ac.
Remove holding bracket off the heater core Phillips screw, several other holding nuts on the firewall,
Remove glove box door and interior box, remove the dash ac control face plate and screws holding the faceplate in, remove lower vents 2 nuts and it will come down, remove vent defrost tubes from bottom, remove the cable attachment through glove box on the top that operates from the control switch, remove vacuum lines attached to the push button controls CAUTIOUSLY 2 clips on each side must be pried apart then pull off, remove wires mark placement for return, slide the control and fan switch through the glove box sorry radio should have been removed prior, under the pass vent box loosen the nut you will see the rod move from inside enough to lift off the vent top edge, the box will begin to drop. At this point for faster removal seat should be out floor shifter if equipped removed, pull towards the rear of the car and downward,once down pull from bottom towards back of car and out towards pass side door. If difficulty arises you can remove a bolt that joins both half’s of the heater core side and the condenser side. That what I remember. Good luck.
