Important elements:
1. when it's traction limited, the torque rating becomes less important.
2. your target speed + how narrow your peak power range will determine how much RPM % drop you can stand on your shift into high gear. As Stan Back said, the other gears are to get there. A bad choice (high RPM power, 25% drop) means it will be faster in 3rd than 4th. Big MPH means load in high gear is mostly aero (in the lower gears it's inertia).
3. if you already have the axle + ratio, OD may be helpful but it uses more power (and higher internal temperature) than direct, and many add-on OD (Gear Vendors 78% recovery) have more drop than you might like. The BSA Gold Star had 92% recovery on the 3-4 shift 60 years ago.
4. if your top speed is really high, you need lots of "progression" (decreasing % drops in the upper gears) to have a useful 1st gear and still do the trick on the 3-4.