Bowzacks

I have a 69 fastback that I'm working on and it’s been a few years now. Saving up and actually have the time to complete some of the smaller projects that go along with this restoration. My question is, and I’m sure it’s done all the time. I would like to narrow the rear to run a negative offset wheel. I know the length of the axel is about 59.375’’ for a mustang so who has done this and what is the shortest you can go without changing the wheel well.