I considered doing this on my 91 crew cab but ultimately never took the time. The housing for a tach cluster has a provision for the back of the tach to poke through for it's stand alone harness. Since the speedo is electronic and the back of the housing is different for it's associated circuit board I figured the "best" way to do it was to use a hole saw to cut a small opening in that housing. After that, you could unpin the big fuel gauge wires from the harness and reconnect them directly to the back of the "new" small fuel gauge.
All that said, the electronic speedo and housing are rare so if you do decide to modify it be very careful as replacements are expensive. Also, the electric speedo doubles as a speed sensor for the computer so that ties in to the auto trans shifting (if equipped) and TBI operation so without it you'll get check engine lights and shifting problems, it would be very tricky to swap in a mechanical speedo.
