I have discovered an effective repair for stripped splines.
Put the lever where you want it on the splined shaft and tighten the screw as much as you can. As 5twins said, increasing the pinch gap helps. Now, drill a 1/8" hole on the same axis as the shift shaft right where the spline is, try to make half of the hole in the shaft and half in the lever. Put a 1/8" roll pin in the hole you just drilled. You have just made a new "super-spline." Repeat twice more in a triangle pattern. Done correctly this will be an effective repair with no play.
Some splined shafts are hardened and very hard to drill into, you may need to use cobalt or thunderbits and start small and work your way up, any you may decide to settle on a smaller final drilled hole size.
I have made this repair one time on an XT500 footpeg which uses a splined connection to allow changing the footpeg angle to suit the rider. Both the splined shaft and the splined peg were hardened. Drilling the holes was difficult. I went through a lot of bits, and I think I even used a small diameter tungsten carbide burr. The final repair was extremely effective. The alternative in this case would have been to weld the peg to the shaft, this is what hacks do, and then they can't remove their engine covers or their engines from their frames. The better alternative is to cut the splined shaft off of the frame, and weld a good used one back on. XT500 and TT500 splined foot peg shafts go for a good price on eBay for just this purpose. In my case I was able to avoid this drastic repair.