Is the 48 on the outside of the crank? Putting a large ring on the inside would bring it too close to the chain stay. Barring that, most mountain bikes have a 42 or 44 big ring, so either of those SHOULD work.

edit: I forgot that you have a road crank. What interface is it? If it's square taper, ISIS, or Octalink, you probably need a bottom bracket with a wider spindle. If it's outboard bearings, I'm not sure how to help that.