help needed with 904

Sealing rings can be different and it's not too big of a deal that some interlock on the ends and some don't. (though I like the iron interlocked ones best)
The trick is to fit them in the bore where they ride and make sure they fit ok.
Basically they are just like a piston ring as far as function.
The outer pressure plates on the clutchpacks can be turned over, but the surface needs to be flat or it will trash the clutches that come in contact with it.

That little feeler gauge is dangerous, and that is why I told you how to make a lip seal tool.
ALL the edges need to be rounded and real smooth.
They do make wire tools like AJ mentioned but I never like those as the were generally a little flimsy for my taste.
Also, you almost can't use too much lube on assembly.
If you think you might have cut a seal, pull it back out and check.

Rings should have a good smathering on them before they go into their bores.
The pump should be packed pretty much full of lube so it quickly picks up the fluid on startup. (most important on used pumps with more clearance than new ones)
The back of the pump seal should be packed full to keep the spring from popping out and going into the pump gears from being seated in the pump.
The pump bushing should be staked AND red Locktite used on the bushing to pump contact area.
Do NOT do this part without asking first if you are not sure.

Oh, and the little check balls in the piston are pressure bleeds.