Actually, Chrysler did offer factory Propane powered vehicles in the early to mid '80s.
At least in Canada they offered it.
My brother had a factory propane powered '86 Dodge 3/4 ton 4X4 360 auto that was run on Propane and Propane only, not even dual fuel.
I remember a few of these around and also cop cars and some taxis with the factory Propane set up.
I never saw an older car that was factory Propane however, but i did have a '66 Chrysler at one time that was run on Natural Gas that someone installed early in the car's life...........