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.
F body rear is also over 3" wider (60.46" wide IIRC). at least around me, anything pre 1990 is just not out there anymore, especially in junkyards. a pre '93 ford Ranger 8.8 will be about the same width as the A body rear (my duster's 8 1/4 is 57.125" wide, early rangers are I think 57.5"), or an explorer with one side shortened to center the pumpkin using 2 passenger side axles (I think that ends up being about 56.6" wide)...never mind, just realized you're in europe