I worked EMS, starting in the mid-1970s. Like many small towns, many of the ambulance services were operated by local funeral homes. Colonial Chapel Funeral Home, based in St. Clair, MI, operated 1966, 1968, 1969 and 1973 Cadillac "combinations" (a vehicle capable of operating as an ambulance or funeral coach), as well as two straight Cadillac high-top ambulances (a 1971 Miller-Meteor and a 1972 S&S). In addition, there was for a brief period of time, an Oldsmobile Vista Cruiser. The elevated roof made it fit right in with the rest of the fleet, though it was already out of service by the time I arrived on the scene. I don't know if a coach builder modified it, or if it was "homegrown," but I understand that it was just too small to work in, as EMS care expanded.
Alas, all car-based ambulances fell by the wayside once KKK-1822, published in 1974 and implemented shortly thereafter, took effect. This is the document that establishes specifications for modern ambulances in the United States.