On a stock GS you only have to shave a bit (4mm) off the mounting brackets of the caliper, otherwise it is plug 'n play. I made no changes to the m/c. I think mine is stock GS 12mm.
Dunno about doing the mod on a G/S but I assume you need a custom bracket.
Some of the above is fact, some is fiction, some is my personal imagination and some is just simple truth. [me]
... i'm not touring around the world, but neither are most of the guys i see running overloaded spam cans ... [bmwblake]
'72 R75/5 The Blues