The 13 inch brake kits I make are designed to work with a Mustang rim. 17x8 with 5.72 fits on B and E body cars. For an A body car sometimes the 5.72 BS fits and sometimes you need to go to the newer rims with more backspacing. I have customers who also run the 17x9 Mustang rims. Sometimes a thin spacer is required.
The 18 inch rims help out a bunch in regards to tie rod and upper ball joint clearance.
Spacer out back is one solution but another popular setup is to use a B body rear end in an A body car. Or just get a custom rearend made up that is the correct width.
