What 8.25 rear axle fits my 1964 Dodge Dart best?

That's correct, but the spring perches are not. They are 1/2" wider apart than the A body. And as I said, they WILL swap in, but why anyone does it, I haven't a clue, since you lose a lot of room on that outside wheel well. Then there's the slight bind the springs are in. It's just not a good idea, IMO, even though a lot of people do it.
yeah, IIRC they're 42.46" due to the splayed (not parallel) rear springs. B body is 43", I had to get a little creative when I swapped a B body rear into my M 20 years ago....cutting, relocating and welding spring perches is easier than dealing with an axle way too wide