I have the Stainless Steel Brakes rear disc upgrade kit ready to install on a '69 Mach 1. Factory equipped with front discs and drums in the rear. What is and if necessary the benefit of installing a proportioning valve to the "new" rear discs.
The proportioning valve you have now has a built in 10lb. residual check valve for the rear drums to keep the pads from totally relaxing from the drum. Disks do not need this and if you leave your old proportioning valve on the rear disks will not release properly and will stay locked up. MP Brakes explains this very well in the Q&A section on their website; http://www.mpbrakes.com