Make CERTAIN the Input Sun Gear is installed in the Input Planet!!!!
Lower the Input Drum/Reverse Input Drum assembly into the Case, You will need to work the 3-4 Clutches over the Planet Hub.....I use a Big pair of Channel locks on the Turbine Shaft splines to turn the drum.
Try not to put all the weight on the 3-4 clutch drive tabs as you work the Drum down in the Case.
The Drum will fall down a little when all 3-4 clutches are engaged onto the hub, The reverse input drum will sit on the SunShell. If no slack/space between the Input Drum & Reverse Input is present, The 3-4 Clutches ARE NOT fully engaged.....Keep working it!