The standard wheel base is the most widely used wheel base and is found on BSA kits, and on wood blocks sold by Pinecar, PinePro, etc.
The centered wheel base is found on Awana kits.
The extended wheel base is available at Maximum Velocity, or can be created by drilling new axle holes or by cutting new axle slots. It will allow a pinewood derby car to go faster and straighter, but it is not allowed in many races. Check the rules for your local race before using an extended wheel base.