Unless you are dropping the clutch at high rpm on drag slicks the 31 spline axles should be fine.
Gearing depends on which t56 close ratio or wide ratio, rear tire height and torque peak of your motor. The wide ratio T56 has a low first and a high 6th so if you ask me no gear makes sense.
Here is one good gear calculator.
http://www.americanpowertrain.com/corporate/Speedo.htm