Best Gearing for 8 3/4 Behind 1970 Duster 318

3.23 is a good compromise gear but a little weak for what your stated use would be. Imo, 3.73 is a little over the top for freeway cruising. That leaves 3.55, which would be my choice.
Coincidentally, I drove a 55 Chevy for 100,000 miles..... with a factory stock 3.55 gear.
(Edit: factory gear for a three speed stick 55 Chevy was 3.70)