The bolt-on perches are offset to the center on each side...that is where you get the 1" extra clearance on each side. They are offset to the center just like both ends of the bar are offset to the center. I have the bolt-on bar mounts on my 69 firebird. I also spent a lot of time squaring up the front and rear wheel mounting surfaces, adjusting the front sub frame, moving the rear side-to-side, etc prior to welding on my upper perches....probably too much time!
http://www.pro-touring.com/threads/1...ar-build/page4
All the measuring fun started at #75