Wiz,
Not all applications are identical, you might ask Ken that question. If the Quick Jacks have been in place for some months, any body preload has reduced.
I recently installed update bumpers. Respecting the SAE I never use less than grade five hardware. All-thread is grade 2.
I welded flange nuts inside the over riders and ran grade 10.9 hex head bolts from inside the body.
Obviously these bumpers and fiberglass will not withstand a crash, but two nuts are twice as likely to loosen as one nut.
jim