A suggestion I have used a few decades:
set the low - reverse clutch pack into the case dry... no planetary unit or other items in the way. Set it up eggzackly how it will operate in the trans. No roller clutch , no race , nothing to get in your way
Set the center support in place , look through the bottom of the case at the clearance. there is an opening there...
A long time accepted clearance for double sided frictions and flat steel plate is .010 per friction. 5 frictions would be .050 clearance.
If you have compressed air ... install the snap ring for the center support and test the apply through both ports
Dont worry how old you are... it is the mileage on your body that adds up.