The only problem I've seen with those brackets is that he frame rail rusts out underneath and compromises the rail. There isn't enough stress there to worry about stripping the bolts and if there was the best solution would be to fully weld around the perimeter for strength and moisture sealing. Through bolts are too easy to crush the rail. While you are at it, make it so you can change the bushing more easily. Essentially make a new bracket design or modify the originals with bolts.