The 8 1/4 thread
I fit a newer Dakota carrier in my 1974/8.25 case with just a bearing change. I thought just line you at one point when nothing seemed to be working or coming together.
But there are so many smart ( a lot smarter than me) folks here so it will come together in sure of it.
So if you have(if not try to get one)a better pic of the posi unit sitting in the case without the caps on so we can get a better look/idea on what you got going on in there.