You can use 2" poly body lift blocks.
4wpw sells them really cheap and yes they would work.
Then you can just go to home depot and get a 2" longer galvanized bolt.
Grade 8 is way overkill for spacing a bumper.
BMW uses rubber bumper bushings and they have a 5 star front end crash rating.
Your bumper doesnt weigh 10,000 pounds so again that is another reason you wouldnt have to use grade 8.
I am sure even with a Warn Bull bar Bronco's never had a 5 star rating.
I could understand if it was a rear bumper and you towed a camper off it, but just for a mount, a galvanized bolt would hold my fat ass with it jumping on it all day long.
EDIT: That reminds me of an accident my friend got in.
He had a ramsey winch bumper held onto his frame with galvanized bolts.
(That was dumb)
Well to make a long story short, he got in a front end collision and the only thing that was still in one piece was teh galvanized bolts.