I would buy one of the billit rear seal retainer offered by Indy and others.At the mating surface between the upper and lower seals put a dab of silicone,be carefull not to get it on the seal lip.Also add a skim coat of silicone to the retainer surface that meets the block and in the side grooves for the side seals.Do the side seals first and install the two side seals,press in to remove any excess silicone and let cure.This will help hold them in place.
After you install the retainer in place take silicone and apply it to the parting line at the outside and at the parting lines at the oil pan rail.
Let it cure 100% before adding engine oil.