I believe you will need 2 of each backspacing. I looked into the R's when I did the M2300K brakes. The 5.95 fronts and 6.45 rears.
Check out my wheels link off my Mustang page for quite a few install shots.
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Darius Rudis
Roadracer: Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page