8.25 to 8.75 swap. is it worth the effort and $ on a street 73 dart 340.

Like in your ex-camp, GM land, the smaller rear is heavily under rated and almost scorned. While the 8-3/4 gets all the glory and love for an EZ change out of the gear set, the 8-1/4 gets forgotten about. Add the C clip issue and a few videos of over powering them, there almost cursed.

Your car also maybe heavier than you think so a trip to the scales wouldn't hurt none. Build and set up accordingly.
You'll be fine.