Sometimes you can find axles with the right bolt pattern, so you can just change the axles. Other than that, you can take the axles to a machine shop, and have them do the work, BUT you have to fill in the hole for removing the axles, and drill/machine another where one of the existing wheel bolts are now.
at the wrecking yard check the big lincolns ,fords and mercs, the ones with 9" rears, the axles have two spline counts, 28 and 31, you need to make sure you have the right spline, altho its easy to change the axle gears to the right ones, email me and i will send you a url with info on 8 and 9" rear ends billyray38@comcast.net