You're right, Terry ... not THAT wide!!
Most pickups of that era (can't say I'm familiar with the Ford you're replicating, so I'm talking in general terms here) don't have seats that meet the door panels (as your model's interior tub reflects). There was generally some distance between the door and the seat's sides.
Having said that, here's how I would fix your dilemma: I would cut the door panels away from the tub's floor, seat and back panel, then use thin sheet plastic to create sides for the seat, then glue styrene strip (of sufficient width to close the gap between the beginning tub and the body) to each open side of the tub, then rejoin the door panels to the widened tub/floor. The end result would be the tub would be wide enough to match the body's interior opening and you'd have a more prototypical space between the inner door panels and the seat.
Just one idea ... and my 2centavos worth.
Worthy project! Model on!