Go ahead and weld in place the axle tubes are pressed in place. Make sure whom ever you chose welds using the TIG process only and verify that the rod he/she uses is for casting base metal i.e. Carbon with nickel. Also make sure this person runs a min of 3/4 inch tacks at 12:00,3:00,6:00 and 9:00 o'clock. Once this is complete the welding can be completed without misaligning the tubes. Also make sure the fillet weld is a min thickness of the housing hub flange casting where the tubes are presses in i.e. If the housing flange is 1/4 inch the fillet weld must be a min of 1/4 inch thick at the center of the fillet weld.