[kwmech]

Using the frame holes as a template I would drill out the bracket than the frame rail. The factory holes are on the edge of 10mm in size. I've always gone to 7/16 fine thread gr. 8 with grade 8 flats and a locking nut such as either a pinch or flange lock, then torqued them down to 70 lbs. I even did an experiment one time with just a regular nut torqued to 70. After 20 years it never came loose