But it looks like you have a ‘65-67 B body rear axle based on that 54” and change flange to flange measurement. If that’s the case, you can probably just use that as is if you get wheels with a bit more backspace than standard. Not sure what your plans are for wheels and tires but it’s definitely doable.
Best news this week! I was kinda dreading all the cutting, straightening, squaring up, welding, etc. Now all I have to do is find someone that's already figured out the backspace thing for 15" wheels.