Any used rear axle you buy used more than likely will require some work, at least to check it out. If it is not the right size for an A body, it will require shortening and perches moved. Why not look for a Chrysler 9 1/4 out of a dodge pickup. You could also get a axle out of a grand Cherokee although its not a high horsepower unit.
I had not thought to look for a 9.25 There is one at the same yard the 360 magnum came from. They want $450 for it. It's from a 95 Ram so it definitely would need to be shortened.