DoctorDiff's master cylinders work just fine with the factory brake push rod, you do not need an adjustable length push rod. The factory brake push rod, with the DoctorDiff master cylinder and adaptor, results in exactly the same brake pedal height as with a factory master cylinder.
I have been running DoctorDiff's 15/16" master cylinder with a factory manual brake push rod on my Duster for over a decade now, I run one on my Challenger as well. Both cars were swapped from factory master cylinders directly to the DoctorDiff aluminum master cylinder with adaptor and both use a factory brake push rod, no change in pedal height.
