disc brake pre72/post73 proportioning valves

After significant searching, I cannot find if the later proportioning valve can be used successfully on an early disc setup. Specifically, using a 73-76 bbp, single piston proportioning valve, on a pre72 sbp, KH, four piston setup. I thought I saw this posted somewhere that it didn't matter but I cannot seem to find that post. Present setup will be early front discs with 9" rear drums.

My concern is the different caliper fluid capacities might need a different valve. If it is not an issue, how late of a valve can be used and from what disc brake car (aspen, volare, cordoba, etc.?) My intention is to presenly leave the 9" rears alone and eventually upgrade them to 11's or disc's when I figure what I am going to do with the rearend. I figure I will probably have to address the valve again at that time. Thanks, dan...