Will a reaar end, springs and driveshaft from a 70 Dart 2dr fit in a 66 2dr?

Yes- all is the same but the driveshaft.
You also will have to deal with the nub of the converter into the back of your engine if you have a V8. A slant your all set and it does not matter. 904 from slant will not work on V8 and vise a versa.