Welcome from North Idaho!
Recommend you start a build thread in this section of the forum: https://7173mustangs.com/discussion-forum/mustang-registry-project-build-threads.78/
You do not need to loosen or remove the dash to R & R the heater box. Removing the carpet helps but is not necessary. Since it is only four bolts, I would recommend removing the passenger seat to allow better access.
Seems to be priced reasonably in the ball park. I'm surprised it is still available.
Given the length of time it has been listed, I'd guess the seller would be pretty open to offers.
I had the flexplate problem that Don C alluded to. I don't know if my old thread will help you, but here is a link:
https://7173mustangs.com/threads/please-help-me-diagnose-vibration-1966-289-4v-solved.41589/