Always a good idea to lube orings and its seating position in diff . Insert orings carefully and make sure they are seated correctly. Lube outdrives shafts this will ptevent tearing when inserting through orings.
Lube gasket both sides to create good seal .all lubing use the same silicon oil as in the diff this will prevent any contamination and you are using same product as what you trying to seal against.
Fill diff to top of spider gears if you want consistency weigh diffs before filling then fill one weigh again and apply this method to other diffs. they should all be close in weight in amount of fluid in them . Move spider gears to settle fluid remove all air bubbles.Apply gasket , line up holes with diff cup.place two tooth pick through to opposite holes .
Slide ring gear over tooth picks this will line up ring gear with holes on diff cup with minimal movement of gasket. Apply two screw then other two remove picks first.
Tighten down in a cross pattern all the way down firm once all screws are down torque them by giving each screw half to one full turn each . See how you go with my ideas hope it helps buddy