I'm just going by the only accepted way of measuring the width. I learned this roughly 27 years ago when I had the 9" rear end built / narrowed for my 66 fastback.

I assumed everybody knows this. If somebody was to order a custom rear end from any of the major suppliers they base everything off of the flange to flange / wheel mounting surface as the width.

Here is how Currie requires that info:

https://www.currieenterprises.com/how-to-order-online

"...and measure the distance between the wheel mounting surface..."