Hurst shifters came with a round L shaped pin that slid through the center of the shifter arms and notch at the bottom of the shifter frame. They probably still sell them separate, but a piece of round rod (bolt shank, etc.) with a diameter just slides through the same way with get you by nicely. I have even used an Allen wrench, you must remember keep the points aligned with the notch and not it's flats, or it won't do you any good. Remember too, if the shifter is a little sloppy from wear, you can do too perfect a job and make it too easy to get reverse when you wanted first. Setting the reverse arm just slightly off requiring you to "jiggle" the shifter a little to get in reverse might be irritating, but beats the catastrophe that results when you get reverse instead of first when downshifting.
If we're talking about an Inland shifter here, like the Hurst, getting the arms aligned is key, but I vaguely remember a tee on the handle to actuate a reverse lockout and connected with a length of cable inside the shifter handle that could stretch or slip.
