An accurate 8 3/4" rear axle width list

I’m about to have an eight and three-quarter inch axle professionally narrowed. Is the 52 5/8 inch measurement “flange to flange”, for my early barracuda, the measurement between the flange backs or flange centers? I’ve seen it stated both ways.

Outside to outside on the housing flanges. And that’s just the bare housing, no backing plates or anything.