I did several things but the slickest was: Hold furring strips in place with clamps, drill a small indexing hole each place you want it fastened avoiding drilling through the skin of the van.... it looks like you made a mistake if you do. Then remove the furring strip, drill it to the size bolt you want. Drill out the hole in the vans supports for the rivnuts, install the rivnuts, return the furring strips to their position and install appropriate length bolts remembering they need to not reach to the surface of the van -looks bad if you make a dimple!