Proportioning valve question

I am doing a disc brake swap on my 70 dart. I have the parts from a donor 73 duster. Unfortunatley I was not able to get the proportioning valve from the donor car. I will be going to a manual front disc/rear drum set up. Anybody know where I could buy the correct proportioning valve for my application?. If I can not find the correct replacement how do the adjustable valves work and are they hard to install?.