Rear seal replacement

Don't worry about it. At least you ask before making costly and troublesome mistakes. Replace the seal while the engine is out.
As someone asked, do you have a service manual? You really should get one, especially if you are going this deep into your car. Get the factory service manual too, not Haynes. It will help you many times in the future.
Use the Felpro gasket set. It's fine. Dennis