Ford and GM commercial trucks have a narrower, flat frame to accommodate standard body builder designs than pickup body trucks.

I would think Chrysler probably follows the same design protocol, so make sure your frame donor is a commercial, not a pickup truck chassis.

I think the frame width is something like 34”, and the original bus frame would be the same.

Pickups generally have a wider frame, partially to accommodate inboard gas tanks, and don’t have flat frames.