Here are instructions for factory seats there should be threaded holes in the floor board to mount either fixed seats or quick remove seats. When you go to the pick a part hopefully you can find a ford van with factory seats it should be a direct bolt in. If you find conversion van seats they are normally bolted to the floor with 1/2 bolts and big flat washers. They will need to have holes drilled and can be a problem getting to the underside to get to the nuts. The tanks or frame rail can be in the way. I recomend trying to find a factory bench seat.
Fig. 1: Lift the handle rearward to disengage the lock strikers
Fig. 2: Lift the seat back to disengage it from the locks and remove the seat
Fig. 3: Loosen the seat-to-floor bolts