Do you have the Cobra Body? Do you have the wheels and tires?
One way to determine the length is to put the tires on the rims and put them in the wheel wells (without axle assy in vehicle), adjust each one, be sure it is vertical and then measure the distance from the mounting surfaces of the wheels from side to side. Then go to one of the axle manufacturer sites and apply the formula shown to determine YOUR finished dimension. Like this:
http://www.currieenterprises.com/ces...mrearends.aspx
The maker of your body/kit may be able to provide the dimension for you based on their recommended tire sizing. The finished length will vary depending on wheel offset.