Set up bearings are great but expensive. You can make your own pinion set up bearing, like Cab mentioned, but you'll want to use a new bearing which means you're purchasing two expensive bearings. Shims under the cup race is easier but requires an assortment of shims made for that cup.

You can purchase a set up bearing through Randy's or make your own, but honing a bearing cone race is time consuming and will wear out a set of honing stones before you've removed enough material for a slip-on fit. I've done it. It'll require you to increase the ID about .004". And this is bearing material!

Another option to all this bearing on and off crap is to use a pinion depth tool. The good ones will cost more than you'll want to spend unless you're doing this for a living, but a cheap one can be had from Summit or Jegs for about $80, Ratech makes a simple tool, but requires bearing removal. #10009 for $29.