Most guys will cut off the outside bearing flange and use a jig to align and to reweld it back to the axle tube after the desired short section is cut out. This is why a jig is necessary to assure that the bearing flange is perpedicular to the axle shaft.
I have done a couple of narrowings by cutting a section out of the center of the tube with a big rotary pipe cutter (rent one from a rental shop). All you need to do then is to clamp the two remaining sections together with 3 pieces of angle iron. The angle iron will assure that the two pieces are parallel. The pipe cutter assures that the cut is perfectly square with the axle tube and a nice "V" notch is left for welding. Then you just weld the two sections together and remove the aligning pieces of angle iron. I think it is safest to tack weld the angle iron pieces to the axle tubes and then grind them free after the finish weld is completed. Then you just measure carefully and order your new axles from Moser Engineering. If you do it this way, you are assured of having good square bearing flanges and even if there is a minor missalignment at the weld suface, the error is split between the inside and outside bearings.
So do it jigless.