I don't know what a 96 frame looks like without the sheet metal, but that school bus frame is going to be very flat on the top from the firewall back. Any variations in the height of the 96 frame after the firewall will have to be accounted for in the floor height.

I had a 64 Doge camper, the floor pan under the front seats, the dash, and everything forward of the dash was Dodge truck. Everything behind the dash, except the floor to behind the seats was all custom built by whomever built the camper. I suspect your bus is built the same way. It may be real difficult to move the body behind the firewall in one piece. It was probably built as a floor pan then the sides and top structure was added, then the whole body structure got covered with the outside skin. Be prepared. Gene