Pulling the backing plate would be a last-resort situation, and that big anchor bolt will be a major pain to remove, as you've already discovered. Get a set of 6-point sockets, not box-end wrenches, and I'll bet the fit will be a lot better. Soak the bolts with penetrating oil like Kroil, PB blaster, etc, and then whack the head of the bolt with a punch and a big hammer to break the rusty threads loose. If you twist the bolt off in the wheel cylinder, no problem because you're replacing it anyway. You can also heat the head of the bolt with a propane torch and apply penetrating oil while it's hot.
Jerry
