Fox body rear end target width for narrowing?

[j-c-c]

Not a mustang guru, my sons 89 track only Fox body is a likely candidate for a TA/Big brake/ Big BC rear axle upgrade. Any body want to suggest a target/ballpark Flange to flange dimension on this 89 car? We haven't picked out wheels /tires, likely 18" custom or common sized, but we are upgrading, and don't see any serious fender flaring in the works, but liberal fender rolling is OK, and realize the bigger tire the better. I also understand factory tolerances, and we will have to measure for final decision, We are dumping the 8.8, and need to start dialing in axle Housings. OEM is 59.25"?, is this best?