Not sure exactly what area your talking about that ya want to shave off the case?
I think you can remove 1/8" of material maybe 3/16" If ya got enough meat IDK
I'm basing this est. on a typical bolt hole thread depth.
Threads usually are 5/8" deep. and the hole is usually deeper than the threads by 1/8" to 3/16"
If the hole is deeper than the threads . You can tap more threads with a Blind Hole Tap
Also if ya got enough meat in the bolt hole depth you can very carefully drill the bolt hole deeper to tap more threads
Do not destroy the existing threads and only drill the hole deeper if ya need the extra depth
If ya use the bit for the tap you should be able to drill the hole deeper without destroying the threads.
To answer you question - Bolt Hole Threads are usually 5/8" but it will still be OK with 1/2" of threads
You should measure the thread depth and the hole depth so ya know what your dealing with.