Is this correct for swapping Master Cylinder from single to dual?

The spring isn't attached to anything, is it? Far as I know on later cars the only "return spring" is inside the M/C. If your pedal is too high it sounds as if you need an adjustable push rod. Putting a newer model M/C on a '66 car is not something I've personally done, but I always thought '66-'72 were all mostly the same. So I'd say contact Dr. Diff... he probably knows more about this than anyone else around here. Mopar '62 -'70 Adjustable Master Cylinder Push Rod