(below provided for future benefit of those foolish enough to try this
Thanks to those that made suggestions. I opted to replace the core. The
Chilton manual is roughly correct except that it contains some unnecessary
steps and fails to point out one key item. After removing the nut from the
stud just to the right of the hose connections there was another
undocumented nut under the evaporator box flange. It was probably just a
convenience at assembly time, but it requires either removing the evaporator
box or cutting off the corner of the flange (my preference). So if you find
the plenum is still stuck after following the instructions check for this.
Also with a helper the job is easier if you can pull the dash back an inch
or so and drop the plenum almost straight down. With the plenum sitting on
the floor and without additional disassembly there is still just enough room
to remove the core.