Front end swap 74 Dart Sport to nonbeak front

could you use 70 Duster front end with the Dart sport fenders?
No. Dart Sport fenders are too long. It might work with 70-72 fenders if the caps are removed.

or go '73 Duster and keep the Dart sport fenders?
No. Dart Sport fenders are too long.

If you want to convert the front end, you need a front clip like BADart shows. Although you may want to make sure the headlight bezels are from the same year. The one of the left looks like a 70-71, the right one looks like a 72.