Most 10" wheels for 05+ Mustangs run a +40mm for the 10" rim width rears. 10.5" +45mm. No "joe dirt" effect on either rim width using these offsets. A few go a little more positive like Forgestar. I am running their 11" +56mm on rear and it barely cleared the inner lining on the passenger side on stock suspension because the body alignment was off 0.25" though they claimed it was ideal offset when I purchased direct. Could actually use about 3mm spacer for some breathing room, but dialing in an adjustable panhard bar got the body centered dead nuts and no issues now even after lowering 1.5". Will only run a 10.5" +45mm rear wheel in the future or an 11" +50mm - +52mm on any S197 I do in the future.
Try beefcake racing. They may have something for you.