front wheel size for 1968 Dart

Are you sure on the 4.75 back spacing? What disc kit and axles did you use on the rear?
I used the MBM disk brake kit # DBK834LXR From Mancini Racing. I mounted a friend 15x7 with 4.25 backspacing and a little to close to fender lip had plenty of room in rear to go 4.75. The axles are stock Chrysler that were shortened and re splined that have the 5 on 4.5 bolt pattern.