Best Gearing for 8 3/4 Behind 1970 Duster 318

As stated above, depends on your goals. Unless you plan on going on with a larger cam than stock and emphasizing out of the hole acelleration, I'd keep your existing gear. You can always find another 8 3/4 chunk and build it with 3.73-3.91 gears. That way if you want the highway gears back, there available as a spare. I had 3.91 gears , 255/60/15 rear tires and mild stall behind a mildly cammed 318 build a few years ago. Super acelleration, but 2800-3000 rpm at 65mph gets old real fast!