The best stock square back is the late 70's 78 amp unit. Its available as reman. The Chrysler case Powermaster ususally rated 75 amps will have similar performance. 50 amps at idle is about as good as it gets with either of these. Depending on what you have now, that may be a worthwile improvement.

With iron heads, the 140 amp Powermaster may bolt in and work. When I tried it on a lowdeck with AC type brackets it bolted up... only problem was the back of the alternator and the Ed heads wanted to take up the same space if it was adjusted in. I returned it and ran a 75 A Powermaster for a while. With a big EFI fuel pump it just didn't do so well - big discharge at idle and barely kept up with AC and brights on running down the road. I finally cleaned up the brackets I had for the "big Chrysler" 100 A alternator from a '78 Cordoba. With a reman alternator, my power problems are solved. Its just heavy.

These cars survived a long time with the idle discharge from new. As long as you haven't added much load and aren't running sensitive electronics, you might be able to just get along with it using one of the better squarebacks. A good big battery helps.