71 malibu w/ power brakes, disk front drum rear. Would a rear disk conversion require new master cylinder or proportioning valve? If so, do they come with disk brake kits or would I have to figure out what sizes I need for these other parts?

They usually don't come with a kit, but it will be needed. There is a good sticky on the rear end conversion on here.

Davoaz, I did the full drum to disk conversion on my 70. From all the searching I did, I would say, yes you will need a new master cylinder. This is because you need a larger reservoir for the rear disc's. As far as the prop valve, I completely removed mine , and put an adjustable one in, and just got some brass brake parts, to connect the lines where the prop valve used to be.

Can anyone tell me what size disks come factory for the front? I hear you shouldn't put a larger disk in rear that whats up front. Most rear disk set ups I've been checking out are 11 or 12". I suppose if I can't match the front size then I'll be doing a new front as well.

NM, sounds like stock is 11".