The doors appear to have an air gap at the bottom to clear (and then
some) the carpet, so maybe HVAC was a concern. I appears possible that
80" doors would fit my doorways IF I had tile instead of carpet plus
pad. Whoever suggested that the house used to have shag is correct.
It DID used to, but not anymore. I'm a little worried about having to
chop the bottom off a pre-made panelled door, as the bottom usually
contains a crossmember support.

Oh well, I'll add this to my list of oddities about this old house...