Spacer is your quick-and-simple solution.

Plan b involves mounting a dremel-type tool with a cutoff disc to the left-side crankarm and using that go around the shell to cut a ring off, repeat for the other side.

You'll need a fair number of thin discs, and you'll have to arrange the mounting so that moving the tool inwards does not change the cut line.
I used a bit of plate that the nose of the tool went through, and an arm whose outer surface I knew was 90 to the axle - loosen the clamps ever so slightly, give it a tap, tighten, grind another .2mm into the groove.

When the groove is almost through, pull everything off and knock the ring off; tidy up the surface with a file, and repeat for the other side.

This way you can go straight to 68mm, which might be worth the trouble.