(below provided for future benefit of those foolish enough to try this repair)
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.