The splines on the wiper shaft are fine, and the wiper arm has soft metal. DO NOT try to run the wipers unless the windshield is clear, and the wipers are not stuck in ice.

After cleaning the splines, you can force the wiper arm down on the shaft a bit with a thin washer under the nut. This does not work if you force it down too much.

The next thing you can try is to wrap a few turns of fine copper wire around the splines to take up some space.

These and other cures will work for some time, but eventually you will see that the hole in the wiper arm has grown quite a bit and you will have to get a new arm. Shop around a bit for a good price.
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'96 855R,'64 PV544, '67 P1800, '95 855, '95 854, the first three are mine, heh, heh, 405,000 miles put on 8 bricks