Just buy those Mustang rotors that some guy machines, sells, for big bolt pattern, for the Kelsey Hayes, 4 piston, disc brake system. :dontknow:
My 2 cents, with your question.
Factory 68 Superstock Darts & Barracudas have 4 piston, Kelsey Hayes disc brakes, with a special "hub" (rotors) part number 2880514 for the 4 1/2 inch, big bolt pattern.
Reference:
Authenticity Guide
1968 Dart & Barracuda Hemi Super Stock
Jim Schild with Larry Griffith