The 3 seats can be moved but you will need to make sure the floor is suitably strengthened where the bolts go through (the original position is through a chassis x member which provides the strength). You may have issues fitting them between the rear wheel arches. You may have difficulty getting access underneath due to the suspension etc at the rear. As mentioned you will also have to move the seat belts and relocate them to the rear which will require additional strengthening along the top of the van where the seat belt bolts to.
The other option is to fit a rear triple which is held in place by 4 bolts, all you need to do is drill the holes all the strengthening is there. It's is one long seat and although it can't be removed without un bolting it does flip forward to give you a larger boot space. The centre belt is a lapbelt and as before you would need to relocate your seatbelts to the rear - you would also need a Caravelle metal seatbelt extension bit that bolts to the roof.