Belltech 3" drop spindles and Baer brake kit

[69ersleeper]

Hello, just joined the forums and I've read around some threads on drop spindles, but I haven't found the answer I'm looking for. I have a 69 c10swb I'm doing a front disc conversion and I'm buying everything peice by peice. So I purchased a set of belltech drop spindles 3" and put them on the truck and I'm now I would like to install a Baer 11" ss4 brake system in the front. I saw in there description of the kit that it only works with a cpp 2"drop spindle. Why is this? Has any one ran this set up before? Any issues? Also I read In my spindle directions that slight grinding of my lower a arm might need to happen( I'm ok with that) also it says they don't recommend using 8" gm ralley wheels do to the offset, but they strongly recommend a 5/16 wheel spacer to fix the issue. What do you guys think? I want to keep a 15"x8" front rims but don't mind going to a 16" rim. Is the 15" gonna hit my lower a arm? Any input is helpful! Thanks!