Best Gearing for 8 3/4 Behind 1970 Duster 318

plug your info in here to see what you'd be turning on the highway and if you can live with that
https://spicerparts.com/calculators/transmission-ratio-rpm-calculator
for what your intended use is, you'll be fine with 3.55's or the 3.73's

worst case is that you have some abysmally tiny cam in there and it just runs out of breath real quick