I put this same kit from Scott on my 72 Pre-L Pantera last year and they work great.
I believe the directions that came with my kit said to remove the backing plates for the front brakes.
More than happy to share any pics of the SAAC kit on my car or to answer any questions you might have if it will help.
Feel free to send me a PM if you would like and I can send you my phone etc...
Scott was amazing to deal with when I did my kit so I'm sorry you're having some problems.
I did have to go with larger rims than factory but was told that might be the case. The real issue was the e-brakes just rubbed on the inside of the rear rims and I didn't want to grind down the e-brake calipers to make them work. Scott warned me ahead of time that that might be an issue and I was planning on going with larger diameter rims to get better options for tires anyway.
I kept the factory master cylinder on my car but I replaced the factory proportioning valve with one that I brought from Larry Stock @ Pantera PArts Connection that had the guts removed. Had to send back my original to get a core charge back but it saved me from butchering up the factory brake lines.
Paul