We almost used the 78 Transport ambulance for a casket for a flight nurse years ago, the family decided against it at the last minute.
Due to the small area in the back of the ambulance, we placed a casket quilted cover in the floor, and pushed the casket up to a point that it rested over the stainless steel plate that would normally stop the wheels on a two man stretcher. The quilted cover kept the floor safe from scratching, the casket safe from scratching or damage... and held it in place fine when we practiced with it... It just didn't look good gathered up on the floor under the casket.
In the 78 transport, a normal size set of rollers would not fit...