To get an alignment that is ideal I would recommend making 4 blanks, two of which will have the outside diameter of your axle bearings with 1 1/4 inside dimension and two of which will have the outside diameter of your ring gear or third member bearings, with an inside dimension of 1 1/4 then you can remove the ring gear, install the 2 blanks where the ring gear bearings go, and install the third member and replace the axle bearings with the other two blanks. Next install a piece of cold rolled steel (cold rolled should be within .002 tolerance)the full length of the housing, going thru the blanks. This will give you a true and straight alignment with less chance of getting off center, causing premature axle bearing failure. After completion of welding you can slide your axle bearing fixtures into the axle housing and measure the gap between the axle bearing fixture (or flat) and the axle housing face. The tolerance should be no more than .005 .