I don't think replacing the rear axle is the easiest thing to do. It would be a lot less expensive to order a set of wheels with a custom backspace to fit that rear axle. A-body 8 3/4's are not that easy to come by, and they're really expensive compared to other body styles.
You could also have the rear axle you have shortened, but that would mean new axles in addition to the cost of modifying the rear axle. I think the custom backspace on the wheels would be cheaper.
So my recommendation would be to figure out the backspace you need and order new wheels. Just based on your picture I would bet you have a 68-70 B body 8 3/4. If that's the case, you'd need 15x8's with close to 6" of backspace. Measure the distance from the spring to the widest part of the sidewall on the tire. Figure you need at least a 1/2" of clearance to the spring, and take the difference and add it to the backspacing of the wheels. It looks like you have a couple inches there, but pictures can be deceiving.
