I'm new to the forum and just bought the OPGI drum disc kit for 14" wheels that you refer to for my '65. It should arrive tomorrow and be installed next week. I'll let you know how it goes. I had 4 drums and power. This SSBC kit uses your own spindles and needs a proporioning valve which I also purchased from them for another $63. This $707 conversion kit is made by Stainless Steel Brake Company (SSBC). The Right Stuff kit was cheaper but I had more confidence in this one and did not want to compromise where safety is a concern.
I'm doing springs, shocks, headers and front end and knew it was just the right time to try the drum to disc conversion. I bought the car with worn out air shocks that were likely placed to compensate for sagging springs. So, I am also going with Belltech OEM height Musclecar springs that are not proportional and Bilstein shocks to try to get some clearance for the rear wheels from the fenders. I have been looking to see if anyone has experience with them.