What works for me on checking the frame. Raise the truck, get it off the suspension. Set the frame up on level jack stands @ the front cab mounts and @ the rear spring front mounts. Then with a "level" you can tell if the frame has a "twist". The cab has to be level and build the front sheet metal starting by bolting the fenders to the cab. Has any of the cab panels been replaced? Maybe necessary to drill new hole locations for the cab/fenders. Another trick is leave the hood assist springs off, so the hood sets on the cowl.
