It seems they based this off a 1969 Flxible 111-CCD "New Look" (imcdb includes the new look bit as part of it's name), except they mixed in some elements from a 1967 Flxbible Flxliner (the front windshield shape, the roof design and some other bits and bobs). The devs also moved the rear door from the middle to the back of the bus for gameplay reasons: