Remove the bolts and deburr the holes. Reinstall the chassis mount with the frame inserts in place, again using C-clamps. Insert a bolt through each of the predrilled holes in the U-bracket, through the frame inserts and the holes that were drilled through the inner side of the framerails. The bolts will bottom out against the undrilled inner flange of the U-brackets. As shown here, remove one of the four bolts and pass the 3/8-inch drill bit through the holes in the outer flange of the U-bracket and through the frame insert tube; then drill through the inner flange chassis mount's U-bracket. Deburr the hole and blow out all the shavings from between the U-bracket and framerail. Reinstall the bolt with washers and a Nylock nut, and lightly tighten. Proceed with drilling the other three holes by removing one bolt at a time, drilling the holes in the U-bracket, then reinstalling the bolt. Drilling the holes this way will keep the chassis mount in proper alignment.