Well, I guess it all starts at the chassis contact point, with whatever system is being used to attach the spring to the chassis. In the case of the front spring, it is necessary to achieve a caster angle which should be built into the cross member. I suggest about 6 degrees but any rake of the chassis can steal that 6 degrees away from you. I'm sure many a hot rodder has gotten his caster angle built into his cross member only to discover it has been lost later in the project when it has all the weight and tires fitted. So stance needs to be considered very early in the project. What should be don't when it comes to the rear cross member???