This post has some good info in it:
https://board.moparts.org/ubbthread...r-main-seal-replacement.html#Post2686329

You mentioned you had both OEM and Billet retainers to choose from. If you use the billet one, watch that it has proper clearance so it wont hit the cap as in the post above. I had one that would hit.Ordered a different one and it was fine.
When I re-did my rear seal, I did not use side seals and instead filled them with RTV. This worked out for my situation - no leaks. I was doing the job with the engine in the car and with trans attached. I mention this because if the engine is on a stand and your doing it there, I think you can see if the side seals are sliding in correctly and bottoming out. Plus you can then seal the sides again from outside because trans is not there. But while engine is in the car, you loose that ability.