This is not an unusual problem, when using that type of steering set-up. Some of the "sites" that sell the slingshot style steering arms even carry a warning of exactly what you are experiencing. To solve it, your best bet is to cut a slot in the outside edge of the shock mount, bend it ( the shock mount) in towards the fender about 1/4" (or as much as you can) at the top, then weld up the slot.