The machining is required, the caliper brackets go from ~1/4" thick or so down to around .1" according to the article. That's more than can be adjusted out. Liberty Rear Disk Brakes for A-Body
I remember reading the articles when they were originally published in the magazine. One thing that jumped out to me was the machining required to bring the thickness down to 0.100. OK, not so bad if the bracket is steel but it is cast iron. A thin cast iron section is not a good thing when subjected to tensile loads. I remember thinking they are opening themselves up to liability issues.
I just re-read the part one article on the web-site and references to particular dimensions have been edited. For instance the .100 dimension now reads *****. Some other specifics have been changed too. Looks like Ehrenberg is trying to cover his butt.
I just obtained the factory brake assemblies to do this swap but I'm going to rethink. I can go to my son's shop on a weekend and do the machining for free. But like Kerndog said above, my 10X2-1/2 rear drums work fine.
The advantagees to me are they look better behind large open wheels and I'm always thinking about weight reduction and the Liberty discs ARE lighter.
