Narrowing rear diff.

Put the wheels and tires under the car where you want them to sit and measure.

That's the best and correct way to get the rear exactly right on length.

Here's a question close to yours.

What color shirt am I wearing today?

You don't know.

Just like nobody can tell you how much to cut off a rear of unknown length.

As a guess, it's going to be about 50-51" total width