Brian,
I measured the bench seat that I am using for the rear of my Tudor which is the middles seat from a chrysler minivan. Widest is about 40 1/2, slightly narrower right at front as the corner is rounded, then straight edge back about 10 inches before it curves in to meet back upright cushion. Seat cushion is 18 inches front to back seating area. Because my original plan was tight I didn't like the amount of rear tilt with the base so I unbolded it from the base and made my own. I bolted angle iron to the original holes in the bottom of the frame and mounted these to 2 pieces of rectangular steel sitting on the side subrails. Since the pic shows a flat floor can I ask if you have installed crossmembers under it to bolt the seats to?