I have a 10 1/2 inch x 18 on the front of my 72 but, the front is widened. The back space is only 5.4". I could have possibly gone 5.9 but it would be very close to the upper ball joint and any wider would hit for sure. I am using a Howe tall ball joint on an AFX spindle so that may not happen in your case but watch for it depending on the wheel makers lip. It will also really start to cut into your turning radius if you don't do other mods.
This is a pic of a friends wheel off his vette on my car with a spacer in between to keep it from hitting on the upper ball joint.
