You'll most likely end up needing to drill a couple of holes for each seat, then bolt/nut them. Be sure to back up the nuts with large, thick washers so the bolts don't pull through the floor and ruin it. If you're working alone a good trick is to lock vise grips to the head of the bolt, they will swivel around and shove against the seat bracket while you tighten the nut from underneath.
Do note that with reg cab buckets in your ext cab, you won't be able to get in/out the back nearly as easily. Not that it was easy in the first place... Though I'm guessing your '98 is third-door.