I would figure out how much horsepower you're making with the engine first, consider ~17% loss through the drivetrain, then calculate your approximate trap speed. From there it's easy to figure out what RPM you'll be turning, and should let you know the limits on the gear ratio.
On pure guessing, estimating around 400 horsepower, it'd be safe. BTW, 6000 RPM isn't really that high. She'll take it.
