It was on the commercial chassis. They were all built by Franklin for squads in northern New Jersey. I'm told the idea came from a member of Pompton Lakes squad, Bud Smock (who later went on to start his own ambulance-building company, PL Custom).
The red and white one in Nick's photos is most likely still around. It has popped up a number of times in recent years. I actually looked at it with hope of buying it in the 1990s, but it was too much work and too much money required at the time. Of interest is the fact that the original '58 Cadillac taillights, and a short piece of the tail fin, are incorporated into the back of the box.
The interior wasn't much, some cabinets up high above the stretcher and the bench seat. But here's a photo. Also a photo of another, a '56.