Master Cylinder

My 67 Barracuda currently has drum brakes all around. I need a new master cylinder and I know the disk/drum MC is different than the drum/drum. From all I can tell is the disk/drum MC has a larger front res. My question is, will the disk/drum MC work for my current drum/drum brakes? I mean why would having more fluid in one res. hurt anything? At some point I will be doing the front disk brake conversion so I'll need that MC anyway. Also, would I need to install the prorp valve now or can that wait until I do the conversion?

Thanks FABO....