1973 340, want more power.

That's why I gave you that on line gear calculator! Us it! Only you can decide if a 3.91 is the best fit, for you.
These are question that you have to ask.........and answer yourself.

how high rpm am i willing to have at freeway speed
How often do i drive or need to drive on the freeway
while this be more of a daily driver.
weekend cruiser
or drag strip Haller.

IN the end the gear ratio you choose will be a compromise. AKA the best compromise for what you want out of it.
use the calculate and ask and answer these questions.