rear end for my 1964 dart

Even if you could find a 7 1/4 SG rear, it is almost impossible to find parts to repair, or maintain it. Ask me how I know.
I ran 7 1/4's for many years. Street and strip with slicks and nitrous. Never broke one. Did have to replace the clutches, once in awhile. I even had a 5.13 SG piece.