Get some of the long aluminum rivits from the hardware store to mount the flares. The regular short rivits aren't usually long enough to go all the way thru the plastic, welting, and metal. Most all of the Early Bronco houses have the black rubber welting to put between the flare and the fender. Be sure and mount them straight. They look like crap if they are on there crooked. I taped mine up onto the fender with a copuple of small strips of tape. That way I could stand back and see what they looked like and make adjustments.
Good luck,
Blue71