I have a 6245 on the E350 chassis so my situation may be different from yours. I wanted to add a swivel base to the passenger seat but was worried about it fitting so I contacted Gulf Stream. They said I could add a swivel base if my floor behind the seat had a notch in it, which is the small step up from the floor of the cab.
Iím not sure how to attach the picture she sent me. If I figure it out Iíll attach it. But my point is you may want to contact them before spending the money.