Narrowed 8 3/4 questions

On a racecar, do whatever you want, I got no advice. But
ON THE STREET
there is no good reason to run 11x3" brakes on the back, cuz yur gonna have to proportion them down severely, to work on an A-body, to prevent a spin on a panic stop.
Even with 295s on the back, and 10x2s, I had to play with WC size to be able to run zero proportioning.
On the street, with 4 same-sized tires, you might be able to run 9"brakes with zero-proportioning. I haven't tried it cuz I NEVER run 4 same-sized tires, lol.
If you insist on continuing down this road, there are better ways to solve this problem, than grinding stuff.
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For instance, when I had my rear end narrowed, I had the axles shortened a lil extra.
The idea was so that I could center 325/50-15 tires in the factory tubs.
and NO! I could NOT have cut off the perches and moved them over, as the springs were right next to the bearing housings and had nowhere to go.
I installed spacers between the backing plates and the flanges, plus adjusters on both axles; so that I could center the axle in the tubs.
Finally; I welded spacers on the end of my axles, on the inboard ends, to push them out far enough to be centered exactly, and to put the shoes in the right place .......... cuz for 325s there is no margin for error.
That was over 20 years ago.