Thats what happens when Engineers start mixing with grease monkeys....have you seen the guys that work and build these for a living??? LOL I kid. Im an ex heavy duty flat rate mech, I can make fun of us.....however back to your issue.
Who installed the studs in the case? If they are crooked or misaligned you will have problems droping in. I always recheck the studs for square before I drop in the chunk. just sayin. p
I am working on a 3D model of a stock early 70s 9" Ford truck housing. Any chance you could post a higher resolution shot of that so I can read the dimensions clearly?