I think you're on the right track with going with a later model MD power brake system. I have an 81 C60 and it was an straight forward job rebuilding and bleeding the brake system. The big plus is you can get any replacement parts at your nearest NAPA. The only issue that may come up for you is that the later trucks have dual (front/rear) master cylinder set ups. I don't know how your earlier truck was plumbed. Good luck.
