Hi Myke
being the bright spark that I am, and because I had some around and it is easy to work with, I initially made a rear wheel spacer out of aluminium for the fitting of the wider Astralite wheel I was installing on the back of my RGS. Long story short, use steel for your spacer material, aluminum can compress.... In my case causing bearing failure on the rear wheel sprocket carrier, allowing the rear wheel to cock sideways while I was riding the bike at a ridiculous rate of speed, scaring me half to death. I was lucky to stay mostly upright.
now all my wheel spacers of any description on any bike are ALWAYS fabricated from some form of steel, usually stainless these days.
and Keith, as usual, is spot on the money, center the wheel between the forks so that the bike will "track" properly, measure what you need to keep the wheel centrally located without binding up the fork sliders, then install the wheel and discs with the new axle spacer or spacers.
only after you have the wheel rotating freely, and the fork sliders able to go up and down without any binding, then move on to the brakes. Install the caliper adaptors and tighten them down. Then offer up one brake caliper onto the adaptor and see if it centers onto the brake disc. If it does, great, do the other side. If it doesn't, then your choice is going to be whether to shim the discs themselves, or the adaptor or the caliper. Personally, when I fitted the Astralite front wheel to the RGS, after I had the wheel itself centered, I needed to move the discs outboard a bit or the calipers inward a bit. I chose to shim the discs themselves rather than modify the caliper adaptor, I didn't want to make it any thinner. To shim the discs outward is easy, remove the discs, obtain a thin sheet of sheet steel in the appropriate thickness required, trace the back of the disc mounting plate, and cut out the sheet steel to suit, complete with holes. Laverda actually used similar disc shims on various triples, my stock 79 1200 wheels have them behind the stock discs, for example.
I know, probably way more work than you were expecting to have to do, the end result is really worth it, the four pot calipers with a proper master cylinder and stainless steel lines are a huge improvement, but don't forget to add a substantial fork brace to help the forks cope with the added braking load and help keep the fork legs from moving around as much as they will without a brace.
Paul LeClair