There's no single chassis size for a 1998 Ford F350 bus. The chassis size depends on the *manufacturer* of the bus body. Ford F350s were used as the *chassis* for many different bus body builders (such as Blue Bird, Thomas Built Buses, etc.). Each builder would have their own body dimensions, and therefore wouldn't have a standard chassis size.
To find the chassis size, you would need to know the specific bus manufacturer and model. Then you might find the information in the bus's documentation or possibly through contacting the manufacturer directly.
