Now I know why 904 was having issues

Now that's creative.But it may be a PITA to install the pump.I find it easiest to have the top of the case a little below waist height, to install the clutches and front end. Use plenty of silicon O-ring lube on that big rubber O-ring,and on the lead-in chamfer.If the pump halves are well lined up, you can almost feel the cast rings entering the bore.Just make sure the front clutch is fully engaged.
When it is right you can feel the clutch thrust face rocking on the T-washer. I pre-assemble the clutches together,on the bench and then transfer them as a sub-assembly,into the case. Some guys count the discs as the front clutch drops down, but I've never had much luck with that.Sometimes the plates are so well lined up, that the dang front clutch drops down past 2 or 3 discs, and then what? No, I'm not backing up and starting over.A little wiggle or a little twist, and poof!Then the rocker test proves it's right.
When you drop the clutches in, you need to keep them together.I usually drop them in with the front band on,too. It is all too easy to snag the front clutch on something as the pack goes into the case.So I push the pack down with one hand and pull the rear clutch up with the other, as I carefully drop it all down. Sounds complicated, but really it isn't. When it's all finally down, repeat the rocking test.Engage the KD band, center the cast-rings, and move on to the pump assy.
Good luck!