Well, 4.5" backspace is a pretty fair amount. Typically the max you could ever run in a 15" wheel is 4.75". With that much backspace, tire width can also become a factor. I've run both of the above back space sizes with 235/60 and 255/55 and not contacted the frame, so I can't say thats your exact issue.
However, I am unfamiliar with the scarebird disc kits. I beleive they mount on your drum spindle and use a ford rotor, so they may create a different track width than stock. Even if these are .25" narrower than stock, you combine it with a high backspace rim and a wide tire, and now you have contact. Additionally, since this kit is a combination of parts and products, the travel limit tabs may not work as intended compared to a factory source set up.
If you want to limit tabs to eliminate this problem, you could do work with this to create a limit point. How exactly you do that, could vary from welding up material, to adding a bolt on spacer at some point.