The older panel trucks were very utilitarian. It would not probably have had a headliner. I'm sorry if I have somehow offended you, but I do this for a living, and what you're suggesting would be more work with fewer design possibilities, and less possibility of success. I'm not saying what you suggested would not be an alternative if he was doing a car instead of a panel truck.
The head liner would not be done on plywood, the base for the headliner would be plywood. This would give him the opportunity of doing the headliner in three or four pieces. The headliner itself would be done out of the vehicle on PVC foamboard and then glued to the 1/4" luan plywood base.
No one lives forever, the trick is creating something that will.