Will early-bay low-backs fit on high-back tracks in a '75?

I think so. I suspect the seat runners are the same from '71 to '76, but it may be different for the later imported Kombis in '77 and '78 that had no upright panels behind the seats. Dunno, never owned one of those.

Are your stock seats like mine? My driver's seat sits on runners and can slide front and back, but the passenger seat hooks onto the floor (choice of two slots) and at the rear of the seat upright.