Dunno about a Cub arm, sorry, but I'm sure someone on here will know. The clearance you need isn't much, most people do a mod involving cutting the arm and stepping it out, I've got away with it in the past by just cutting a piece out the inside of the right-hand arm, where it tapers in and tyre would catch, and plating it up, then adding a curved plate along the outside to retain the strength - not particularly elegant but cheap and cheerful, it works fine on my Bantam framed Sunbeam. And yeh, overcoming the hurdles is definitely part of the fun. :-)