I just did this swap on my 72 K20 but I went to a disk/disk setup. I have some parts left over from the conversion that I am willing to sell. A Classic Performance Parts (CPP) 1972 Disk/Drum Master Cylinder with all new lines to the Prop. valve and a new bracket that holds a new CPP Disk/Drum Proportioniing Valve. I also have the proper length MC rod (shorty) that goes with the 11" power booster and this MC. This is for a C20 or K20 with disk brakes in front and drums in the back. All of these parts are a little over one year old and work great for disk/drum trucks.
Give me an e-mail and let me know if you have an interest, I'll let it all go real reasonable.
Thanks,
72longbed
suprtuff@fast.net