Dialing in my 73 project Duster

A 7.25 rear end will not last long behind a 340. I grenaded several of them with a 318 4 bbl. You can put in a Ford 8.8 rear axle from an Explorer with a little bit of fab work, but they typically come with 3.73 or 4.10 gears, and a traction lok differential. Use the search engine feature here to find more information about the swap. An 8.75 a body rear end will not come cheap, and an 8.25 may not be stout enough, depending on your actual power output. A 9 inch Ford won't be real cheap either, but it will probably be less than an a body 8.75, and it will take all the abuse you can throw at it.