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

Dart sport has the pointy front. You would have to swap the whole front clip over, including hood and grill. Still not positive it would bolt right up but you should be able to make it work.
The outer stamping is identical from 67-69 and from 73-76. The attachment points are too except for the flat area at the core support. The headlight bucket on the later ones sit slightly different so as stated, you can drill out the spotwelds and transfer the early buckets over to the later fenders.