What 8.25 rear axle fits my 1964 Dodge Dart best?

Just remember, with the 8 1/4, you'll be going from the small bolt pattern to the big bolt pattern so you'll need different wheels for the rear. Also, the F body 8 1/4 will "fit", but the spring are like 1/2" wider apart. You can just push the springs out a little on each side and it will fit. I don't endorse that, but plenty of people have done it.