i think fireguy's method would be the easiest and almost as good as welding in a new piece. if you put large (enough to cover the open seam by at least 1/2") washers on either side of the floor, and then a new bolt thru, it will spread out the stress from the seats much better than just the bolt. You could primer the seam so that it doesnt start rusting. This should stop further damage, and in the future if you have the chance, have it a new floorpan welded in.
What model year is this? Ford added a reinforcement plate to the floorboards in early 90 to address this problem. Unfortunately, it only does a halfway decent job of stopping the problem. Mine failed when the captive nut broke loose from the reinforcement plate.
Adding a washer helps, but it doesn't stop the problem. The pan still flexes around the bolt, and the tear still gets bigger (as much from body flex as anything else).
Remove the seat, drill out the crack, weld it up. Add a reinforcement plate and washers to help distribute the load over the thin floor sheetmetal.