I've got a pass through bench seat from a 79 full sized Ford passenger van, I'm putting in my 46 Studebaker roadster pick up... the Studie had an under seat tank with a riser, & if I welded new feet on the seat frame, it would have gone right in, 1" to spare from inner door panel to inner door panel...
I'm moving the tank, & removing the riser ( in fact my floor will be sunken angled to the rear, to counter the steep rake the frame will have )
but I'll still be using the van seat... big enough for 3 if needed, & pretty comfy