Might look into presure wave tuning for intake manifold design. I've got a spreadsheet that I set up to determine the dimensions for my intakes but it calls for Engine specifications like Advertised Cam Duration, Desired Tuned RPM, Intake Valve Diameter, etc... But there is plenty of info to help determine what a ball park length for the runners would be if you really look into it.
Basically if you do it, you will lose power below 6K RPM and gain above it. So, unless you are geared around 4.30 or higher and like spinning the piss out of your motor, it's best to leave the stocker alone.