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.
Sweet!
I'm Gona try and take it on the highway asap. See what rpm I can tolerate.