7 The IRS saddles dovetail onto the rear framerails. But first, measure 24-inches from the rear leaf spring shackle mount (shackle bushing bore in framerail) forward to this point on the framerail. Make sure both sides measure up the same, keeping in mind there may be some past accident damage. If your framerails are not true on both sides, don't proceed with this installation. You must have spot-on framerail dimensions. This is where your Heidts saddle should be positioned. Before drilling or welding, double- and triple-check your measurements.