Sounds like Mr. Brewer corrected all thing on the transmission that was an obstacle and, on the spot, and fairly priced....the length of the input shaft will need be addressed and likely a shortcoming of the supplier of the adapter it would seem. Looking like a spacer would need be machined for the front of the transmission and adapter. Typically, I think they are more concerned with the later 833 with the OD gearing in their fabrication which is the larger cover and different input shaft (?). I have the OD 833 here on hand but have never measured any physical feature to verify just what would be the difference between it and the one you have. The answers would likely need to come from the AOK boys who sold you an adapter and may be best directed back at them instead of here. You almost there. to prevent chatter your spacer will need be true in thickness...likely a milled/ground surface.
As an added, am I correct you did not notice the length difference till after your trip to see Mr Brewer, they have all the different input shafts on hand and he could well advise I would think with a phone call as to what input shaft you may need and or if there is a difference in overall length. I know the one I placed in a setup many years ago had the pilot bearing length trimmed down to allow it to snug up....could be an option here.