Rear M Caliper on a 68?

[Sweetheart912]

I observed a 68 has a big piston & pad M caliper in the front & the older 356 style, small piston caliper in the rear. A 69 has big piston/pad M calipers all around. Any reason a 69 rear caliper won't just bolt up to a 68? The rear rotors on a 69 are different only because of the shape of the "hat", as far as I have seen ,& does not use factory spacers as a 68 does. These two things should have no effect. It does have the same 10mm rotor thickness & other specs, & uses the same dual circuit MCyl.

Is it do-able or is it something as simple as the bolts don't line up? Has someone actually done it vs theory? I dont have a rear M to check out to compare. Thanks very much.
George