For 1968 the Muncie 4spd backup switch bracket and reverse lever was one year only. These parts are very hard to find and locate. There use to be someone reproducing the kit but I have not seen it around since Performance Years was taken over by Ames Performance.
The Backup Switch and retainer is the same items used for the 1969 Camaro Cowl Induction switch so these items can be found.
List of parts:
Backup Switch – GM 9790771 or Replacement GM 3959115 can be found as NOS or Quality reproductions. You can find one here. Switch
Retainer Clip – GM #9788460 can be found here Retainer Clip
Backup Switch Bracket – GM# 9792096 a reproduction can be found here Backup Switch Bracket
Reverse Lever – GM#9792026 – I have one available to purchase. Other then that searching forums and ebay is couple other places to find them.
Best place to search for the bracket and levers is User group forums, Ebay and other used Pontiac parts dealers.