Heres my thoughts when I went down the path of trying to convert to disc brakes..
The original disc brake thread can be quite confusing.. at least at the time it was for me. I did end up finding the correct P30, big 5, front axle but have yet to machine the sleeve spacer (I think thats what it was?) and install.
Mostly because I also wanted to convert to hydroboost at the same time. I've got a hy-power (hydroboost) unit and master cylinder from a GMC 7000 that I thought I would use. However, a member on the 67-72 forum tried this setup and noted that the brakes were pretty stiff. I guess the pedal ratio of the old setup did not match up for the hydroboost master cylinder. If I recall, the brakes worked but not great. Maybe one day I'll figure something out myself.
If I could go back and advise myself, I would recommend to go with Randy as he has said he uses common parts. That and hes in Indy
