Best Gearing for 8 3/4 Behind 1970 Duster 318

All depends what your trying to do, performance wise I feel you should run like 2 gears deeper than people with a similar built 360 generally would, so a 360 like yours generally gonna be 3.23/3.55 so to me 3.73/3.91. If highway matters more or best 1/8, 1/4 mile or best 0-60 mph or 0-100-0 mph all gonna change what to run.