Axle conversion?

OK I am totally confused, I have an 8 3/4 axle off of I believe an 68 Dart Swinger, it has the sbp pattern axles, I have measured the backing plate from end to end and it is 11". I want to switch over to the bbp. I have called doc. diff to make sure I get the correct axles, he informed me about the backing plates needing to be switched over. I have been looking for a set of backing plates and after reading I am confused as to what I am looking for. I have read about 10 1/4 10and 2 offset this and that, So if I can get some clarification, with the 11" backing plates I have do I even need to worry about and if so what do I look for? thanks a lot in advance