Rear end

Hi how's everyone doing today?
I'm interested in upgrading my rear end, which way would be the easiest to upgrade to an cheapest? I don't mind doing modifications an what not , I have a 72 duster with a 5.9 Magnum swap an transmission is a 904, this would be all new to me ,I'm learning as I got

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Why do you want to upgrade the rear? Different gears, current rear worn out? If your current rear is in good shape you might want to upgrade your torque converter instead.