Which 7/8 master cylinder?

Not to me.
Did they give a reason for their backwardness?
But internally,the pistons are or should be,the same size; so the calipers won't know the difference.
But you will have to plumb your Combination valve differently.

But on a disc-drum system, I would sure want to plumb it the way Chrysler did.
No they didnt and I should have asked I guess. but I agree with you.