I think one of the hardest parts to concour when learning to rebuild 904/727 is to not force the front pump in the case and breaking a steel sealing ring. Or at least it was for me.
If you use aligning studs when putting the pump in you can wiggle it back and forth a little going in to let the rings kinda jiggle in to place.
They can hang up on a machining ridge really easy.
One thing I noticed we never really mention when we help people with trans stuff is that we all seemed to forget to mention assembly lube.
It's critical with teflon rings to keep them in place until they are in thier bores, and all the bushings need that startup lubrication until the fluid gets circulated.
It also makes seating the lip seals to the drums and pistons a lot nicer with less danger of cutting a seal.