I went back out to garage with new determindation after reading all the posts that Marty referred me to (see above).
They nut is indeed hidden behind a metal panel that appears to be part of the original dash/cowl assembly.
Anyway, with new purpose of mind, I did manage to get a 1/2" open end wrench (called spanners here) up into the cavity and managed to remove the mirror bolts.
That was the easy part. I have no idea how I am going to re-install it when the truck comes back from paint!!!
Two leasons learned:
1. don't give up on the first or second try
2. old bodies aren't meant to contort like this anymore. I think I hurt something.
