Get the short leg, but also ask for triangular feet (they're not an option on the shop page, put "triangular feet" in the comments box) - all the Jacksons I've BKP'd had quite a small rout that the square feet didn't quite fit.
If it's under a mounting ring you can just widen the hole by ~˝mm with a dremel without altering the external appearance, but you might as well save yourself the trouble & get BKP to make a pickup with feet that will fit :)