4 Spd driveshaft

I need to know the exact length of the drive shaft for a 67 Barracuda with a 4 spd cast iron OD and a 8 3/4 rear. I have a used drive shaft thats going to be cut and balanced and want to get it right the first time. How is the measurement made, from the center of the universal joints? Thanks for the help, I'm just trying to get all the parts together for this spring.:-D