What 8.25 rear axle fits my 1964 Dodge Dart best?

An 8 1/4 out of a '73-'76 A body will be the correct width for your '64.
HOWEVER-
-You will need new shock plates and U bolts; the axle tube diameter is larger than your 7 1/4.
-You will need to shorten your driveshaft or have a new one made; the 8 1/4 has a longer nose, so it requires a shorter driveshaft (IIRC it's about 1 1/2- 2" shorter, do some searching for the exact number- there's lots of threads on this). Your '64 uses a ball and trunnion transmission output instead of a slip yoke and it may be difficult to find a shop that can still build a new one, so shortening your existing driveshaft may be your only option. There is a mod for building a new shaft, detailed here: https://www.earlycuda.org/techdocs/ConvertFrontUJoint.pdf
-With the 8 1/4 you will have the 5x4.5" bolt circle, which won't match the 5x4" bolt circle you have on the factory front brakes.
-The parking brake cables are different, but can be made to work.