Center to Center spring perch is the critical dimension.Face to face of hubs also critical depending on wheel offset, if any. 
     We replaced our 7k  12" x 2"   brakes Dexter axles with  8k. forward self adjusting 12 1/4" x 3 3/8 shoes ordered from Redneck Traler supplies with perches loose, welded them on myself with pre and post heat to minimize HAZ  ( heat affected zone ) so I could vefify alignment with lasers.  Tire wear and braking much improved,. 40 k mi., no issues. 
  If you do mot have the ability to do it yourself, consider ordering the axles with loose perches, then have a local shop weld them on.  Welding  procedure to minimize stress, warpage will be superior to the factory.