The 14 bolt "bible" does have a lot of good info but for as many questions as it has answered for me it has left just as many unanswered...

The bore in the backing plate does not fit snug on the axle tube diameter. It does fit snug over a larger diameter pilot turned on the flange. If spacing the backing plate out I'd want to again register the center of the backing plate on a pilot, the spacer would have a male and female side to allow this.
Here is a picture of the flange, and measurement 5-15/16" from back of hub to flange which is for the deep 13x3.5" drum.
PICTURE

Here's how not to do this and you can see how the backing plate does not fit the tube but would fit a larger diameter pilot.
PICTURE

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-