seems easy enough, narrow the arm, press the bearings further in and make two top hat spacers, i'd make the spacers from a decent quality steel, the outside dia to suit the bearings, the inside reamed for the spindle and stepped at the end to locate in the crush tube. it'd be best to have it all done at once, have the modified arm sitting as the new top hats are made, that way it can be assembled and checked that the space between the top hats is enough to stop the arm itself being nipped up and stopping it rotating around the spindle
does that make sense ?