IMHO. I would use the MC for the donor car. So if you got your BBP front brakes off a 74 I would use the MC for that application.
If you go disc in the rear you will need to remove the pressure valve out of the drum brake side of the MC. As for proportioning valve for rear discs, start with what you have then decide if they lock up too easily. If so get an adjustable one and start testing till you have a hard time locking them up.
I put 4w disc in a 56 f100. I had a adjustable proportioning valve and had to adjust it to it's max to keep from locking up. It was like 10 to 1.