Does this panel truck have supports across the roof from the cab back? Could you take a picture of the inside? The best way to do a "hard" headliner is to attach a piece of 1/4" luan plywood to the roof supports to be the base and then glue PVC foamboard panels to the base with top and trim contact adhesive. This could be done in two or three sections.
No one lives forever, the trick is creating something that will.