Cudasteve, from your description it sounds like you have Amarillo Phase V ceiling panels,very common ceiling material used on units built in the late 70's early 80's.
These were made of a cardboard/cork/paper composite mystery material (picture dropped ceiling tiles) installed parallel to the trusses and were wide crown stapled to the trusses.These were 4' wide with channels at 16" OC or 24" OC the plastic splines were inserted to cover the staples. No way would I mud and knock down on top of that stuff.I would overlay with 5/16" or 3/8" gyp,securing it with screws . I would not use 1/2" gyp due to the weight and the possible overload of the bottom chord of the trusses.
This would be fairly big job but not overly complicated, a lot of little things like wall trim, ceiling trim, light fixtures etc possible ceiling grilles if you have overhead ducts.