I am making plans to build a disc brake conversion kit for a 1980 model 14 bolt ff. I see that most all kits have a set of spacers that space the backing plate away from the mounting flange on the axle tube. Can anyone tell me the length of this spacer. I plan to use 3/8" or 5/16" plate and front rotors from a 3/4 ton chevy 4x4. Please help.