Great looking car!
My guess is the door panels from a 1968 Catalina, Ventura or Bonneville two door hardtop will physically fit, but the problem is they have vent windows and the Grand Prix doesn't. As a result the door panels from those cars have a hole for the vent window crank.
For reference, there is a bunch of factory literature including a colors and interior brochure at the Old Car Manual Project website:
http://oldcarbrochures.org/United%20...iac/index.html
Bench seat GPs aren't common but you do see them once in a while - I used to own a 1965 GP with a bench seat. Mine was two tone with a black painted roof (not vinyl.)
