Can '73 Dart Sport fenders be used on a '68 Dart ?

The basic exterior stamping is the same from 1967 to 1969. For 1970-72, MOST of the fender is the same too except the leading edge of the fenders had a pot metal end cap. In 1973 and until the final year of 1976, the exterior stamping returned to the 67-69 style.
The horizontal/flat portion that attaches to the fender apron has the same bolt hole location. The metal panel that attaches to the radiator core support is slightly different and has attachment holes in different places. The 67-69 fenders can be used on the 73-76 cars and vice versa if a few small changes are made. Again, you may need small bits from original 68 fenders to make it work.
This same situation applies to E body fenders. Year to year changes can be reversed or changed to save a few bucks since the later ones are usually cheaper to buy used.