Watch your chainline. The big ring on standard doubles is usually about 46mm from the centreline if the frame. Standard hub / cog combos give a 42mm chainline. Well, they're supposed to, but check Sheldon Brown's
chainline article to be sure. Or just measure.
If your chainline is off by 3mm or more, you'll want to correct this by either mounting your ring on the inside or getting a BB with a shorter spindle. The first method may not work, as the chainring may hit your chainstay. The second method may necessitate grinding off the shoulders of your crank (i.e. where the inner chainring would go).