I too have a 68 Overlander and have gone thru it quite a bit, including the AC removal you describe. When I install my nes a/c it will be thru a new 14" sq opening.
If you are doing a remodel of the interior, you might find that the interior skins have to come off for electrical rewire, relocating outlets, re-insulation, etc. My interior a/c blower had to come out to remove rivets from the interior ceiling skin. I was a little reckless with my old unit and cut and hacked so it was not reuseable. Please let us know how the preservation comes along.