8 3/4 swap help please!

I am getting ready to have a 70 b body 8 3/4 narrowed for my 69 dart have a 742 case cone type sure grip. purchased 11x2 1/2 brakes have offset spring kit in already. getting ready to buy axles .Need any and all input,Please. Also what is the problem with the brakes something is supposed to be larger or thicker between the both that causes problems. pleeeeease Kevin.