Upgrade for '70 Swinger 340 rear

Get a truck dana 60 have it narrowed and set up with a spool or lsd + 35 spline axles. 3.54 gears .
Calvert split mono leaf springs. Assassin bars or caltracs. Don't worry about noise, your not going to hear it over your engine and it's a mechanical noise so it's cool. You have your mustang for comfort and quiet.
drive it with the 8.75 until you get your dana done.
have fun and I hope you're enjoying Florida