I use permatex gasket sealer on this. I put the gasket on both sides of the inner cup, and let it dry overnight, that way the gasket won't move. Then, put sealer on the back of the transmission and install the cup and hold it in place with two bolts, one on each side, with the nut towards the rear. The rear end can now be moved into position, the drive line inserted, and moved forward as much as possible. Remove the two bolts, and finish sliding the rear end in place. Put sealer on the bottom clam shell half, and install it. At this time I pump the cavity full of grease, you will see it come out the inner cup holes.
Sealer on the top clam shell half, and install it as well. Tighten all the clam shell bolts, then install the FAM .
Just my method, but it works for me. Just did it a couple weeks ago installing a Mitchel OD.