Need brake help, Please

[leebo1082]

I've been lurking on this site for a long time, but I think this is my first post. I despirately need some help on the project my dad and I are doing. We've decided to do the 2012 Power Tour in our '61 pickup, and to make it tour-ready, we decided to put six lug disc brakes on it and lower it. I believe the truck is on a 64-66 frame, since it has coil springs up front with drum brakes. I did a lot of research prior, and this is what we rounded up. 1971-72 1/2 ton 2" drop spindles, 1971-72 1/2 ton calipers, and 1991 3/4 ton 6 lug rotors. We installed new upper and lower ball joints for a '71, and the spindles bolted right up. My problem is that my dad just called me and said that the calipers will not bolt up. Apparently the piston is making contact with the inside of the rotor (no pads installed yet). Our six lug rotors are the same thickness as what is called for on a 1/2 ton '71 (1.28"), so I can't figure out what the problem is. Any and all insight would be greatly appreciated! Thanks, Lee