I would take it to a machine shop and have them grind the spacer instead of trying it myself. While the instructions are similar, they aren't identical. Notice that Moser doesn't even mention shimming: http://www.moserengineering.com/aitd...aitfile_id/88/
You have 60 thousandths for tolerance and your spacer can't meet it? Unless there's something way
strange about your axle, if it was me I'd return the axle set and order some Mosers or other quality kit for about the same price. I believe Superiors are gone now, though. Frankly, if you're comparing your axles to Crown or Omix, you're not saying much. (But for I know you pulled those names out of the sky for comparison) Yukons are supposed to be good as well.
To their credit, it does look like they're using Timken bearings, but it's the spacer sizing that matters.