.....I had the bottom sides of my fender flares sanded smooth, and a slight roll added added at the edge to simulate an aluminum fender. Did this front and rear, and also glued some black closed cell foam to the undersides of the fenders to protect them from getting "stars" from stones hitting the underside.
The foam is about 3/16" thick, and was adhered with 3M spray adhesive. If you check my gallery, you can see the edge of it in the shot of the rear fender, and also in the shot looking up into the front wheel-well.
My FIA car has "thru-body" exhaust like a big-block. I added some aluminum plates to close off the gap between the footbox and the frame. I also added some pieces, backed up with styrofoam insulation, under the footboxes, too.
regards,
Jeff Burgy