Will the rear axle fit?

They're not the same size, but a '68 B-body 8 3/4 will work in a Dart Sport or Duster body. I have a 68-70 B body 8 3/4 in my Duster. You do have to move the perches, the B body perches are at 44" c-c and will need to be moved to 43" c-c. I moved mine to 42" c-c and installed a 1/2" spring offset kit, figured if I had to move them I might as well gain some room for tires.

The '68 B body 8 3/4 is wider drum to drum too, so you'll need rims with more backspacing. Not a great thing if you want to stay with 15" rims, but if you want to run 17" or 18" rims you'll have plenty of off the shelf options with the right backspace.