Now I know why 904 was having issues

I use a feeler gauge ground down to a taper with smooth sanded edges for installing lip seals, but almost anything will work with a little patience.
You just don't want to trim anything off the rubber when it goes in the bore is all.
Even a rubber band around the lip seal and stick the piston in the freezer for 20 min works pretty well.

On the lip seals, if there was a question about which one goes where I would put them both on their pistons and see if they have about the same tension.
Stretching them is not a good idea because the sealing edge could get pulled away from the piston bore.
I have seen some that were a little sloppy on the pistons be just fine, if that helps what you are seeing any.

On the discs, are you pretty confident that the old ones are thinner due to wear?
If you can get all of them in the drum and still have the clearance you need then everything is fine there.
If not, you may have to do some creative stacking as I have at times had to use 2 steels side by side to get tight enough, or shave the pressure plate or even the piston for a drum to get the clearance I needed.
Or if I had a lot of clearance, I might shave the pressure plate AND add another disc and steel.