No endplay adjustment is needed for the reverse idler gear. That gear rides on that large stubby looking shaft that seems to do nothing in a 3 speed gearbox. The other gear, the reverse gear itself, takes the place of the #2296-33A mainshaft spacing collar. End play is adjusted on the mainshaft as with all 41-73 transmissions. You will however need a reverse shifter gate. Your shifter lever gear is marked for 3 and R. As a 3 speed, the 3 on the shifter lever gear is mated to the beveled gear on your shifter drum. To change the timing to a reverse style, you mate that beveled gear to the R mark on the shifter lever gear.
More information on reverse gear transmissions can be found in Servicar service and parts books.