Nutserts would do a pretty good job, I wouldn't put any type of "self-tapping" fastener there. FWIW, My aftermarket Hotchkis brackets are welded in.

If you are through-bolting it, you want to add some compression sleeves inside the frame rail to avoid crushing it. Drill ~7/8" holes, insert 7/8" compression sleeves, install bracket, put 1/2" bolts through it.