I've removed the evap core and heater core on a previous 73. No need to remove dash at all. You might want to remove the cardboard glove box for better access. Info on doing it is in the FSM.
A word about drainage, fenders, cowls and leaves... I lost my original 73 wagon to rust in the cowl and unibody where the fender meets the unibody "frame" due to leaves and the resulting poor drainage. By the time I figured it out it was too late.
Check the cowl first. The vertical surfaces under the windshield were difficult to get paint to trough the two vent/access holes on the top. Likewise, leaves and debris can clog the main drain under the wiper motor to make for a perfect storm of rust at the weld between the upper and lower cowl panels. Same issue can easily occurs at the base of the fender near the door.
Your car car looks to be in good shape other than the passenger floor next to the vent.
hope this helps.