I tried finding an old picture of my 96 cab floor and only found one that didn't allow me to see far enough back into the cab on the floor. I am thinking that there were holes for seat belts but can't remember...the previous owner removed the seats and there were no belt buckles when I got the cab so they could have been fastened to the seats as well.
If Ford had put in a 60/40 bench there should have been hold downs for the seat belts on the floor...meaning they are predrilled but may not be used.
I checked the on-line manual...
http://www.therangerstation.com/forums/showthread.php?t=319
The slide bar that the seat belt attaches to is apparently bolted to the floor behind the seat. If you use a different type of seat belt that hole can be used to secure it to the floor...
However, just FYI...the bolt they use to secure that part may be different in strength than a different type of seat belt. They are probably the same...but not sure...
The seat belts I am using in my 88 cab had both a slider type on the seat as well as the slider bar fastened in a similar fashion...but I used the original bolts from the 88 regular cab to fasten the seat belts to the floor...these have a bare spot where the seat belt eye can move freely up and down without interfering with the threads.