I am in the process of building a traditional t-roadster chassis - first time building one. I am going to use a speedway medium arch spring on the back axle. Speedway says this spring measures 38" eye to eye, don't know if that's unloaded or loaded. My shackles are 1.5" bolt center to bolt center - this gives me 41" overall width with the shackles parallel to the ground. My question is how far apart do the perches on the axle need to be in order for this spring to work correctly? I have searched and have found as many different answers as there are search results. Hopefully someone can give me a definitive, no shit answer.