When i put ladder bars under my valiant i measured from where the factory perches bolted to the frame back to where i wanted the axle centerline to be in the wheel housing. FWIW, you will probably have to move the axle back a little to fit a 28" tire into a stock wheelhouse as the axle was towards the front of the housing from the factory on Darts. I made 3/8" plates to bolt between the spring perch and the factory mounting location to fit a 28" tall tire under my dart, otherwise the tire would rub the front of the 1/4 panel.