Pitman arm and gearbox issue

I have a 1967 mustang v8 manual steering car. My dad bought it new in 1966 so it is an early model. I needed to replace my steering components since they were pretty worn out.

I bought a Lares 18702 which has a 1" sector shaft which is what my car had from the factory as my tag was smb-a. I bought a 1 inch sector shaft pitman arm using part number c7za-3590-a which according to stangersite that should be my part number.

Now my issue is that I cannot get the pitman arm to go over the sector shaft by more than maybe 20%. Did I miss something in my parts ordering? Has anyone else ran into something like this?