I had an EH diff shortened 1 1/2 inchs either side,
and had the axles done also(obviously one would think)
Turns out, I only had to shorten the housing, and use LJ Torana axles.
By the looks of the pic with the ruler, and the text you wrote,
It sounds like your assuming a 3" short diff = 3" either side?
The LJ Diff & axles will give you 1 1/2" either side.
Love the wheels by the way
How much you shorten the diff, Is affected by right hight you want to some extent.
If you want it low, you'll need clearance on the inside & outside of the tyre.
A higher ride hight, will clear the rubbing easier.
Regards Jeff