Just what is dialing you into exactly 58.0 inches???? Tire and wheel backspacing???? If its wheel back spacing, maybe a different rim width/backspacing could solve your puzzle????????Whats the bolt pattern? Lots of Mustang rears are 4 bolt axle flange
And why just a ford 8.8???/ The Granada/Versile rears were 9 inchers with disc brakes at 58 inches

What suspension method are you using???Leaf springs??Triangulated 4 bar???all these items can direct the choice to sometimes a better beginning--------