Stock Mustang "Bullitts" have 5.72" backspace!!
Just fine when you subtract .25" for added rotor thickness along with either two piece hubs machined down or machined down single cast rotor/hub hubs, (less the rotor), when going from drums to disc's...
I'm using 1988 Vette 13" front rotors on the two piece disc hubs and a custom 1969 big single piston caliper bracket and the wider Van caliper, then had Mosers machine everything to the Ford 4.5" bolt pattern and added 1/2" studs for extra strength.
My rearend is 1989 Camaro 11.75" rear rotors and reworked Cad Seville brackets and again redrilled like the front.
I've looked at this stuff from all angles!!!! pdq67