Wheel base is the best way. Two things to keep in mind. One, you must be the same left and right or you won't track straight. Two, keep in mind the anchor that was part of the torque tube system you will need that to control spring wind up under load. A good reference might be the center of the king pin. It won't move the spindles will. If you can, use the carpenters trick and measure the diagonals. Left front to right rear and the same for the other side. This will show any minor adjustments. Good luck and show us what you have done.
