68 dart Disc brake up grade, what MC?

I used 1 1/32 non-power iron and 1 1/8 aluminum from a B-van on my old Dart and both worked fine. This was with the A-body 2.6-inch piston front calipers and small rotors with stainless braided lines. HOWEVER, you are using a different caliper piston diameter and different effective disc diameter since you're using the GM stuff. So I have no idea what your pedal ratio is going to be.

So not too many people are going to be able to answer your question properly. In general you should use the largest MC piston that you can still push with enough force to stop the car - that gives you the shortest brake pedal travel and hardest brake pedal.

Which one is it for you? Who knows. Hopefully you'll find someone that's done the scarebird conversion. (Not me - the backyard re-drilling of the rotors scares me, honestly)