[will slow gto]

I'm having trouble recalling which style fastener to use re-installing the splash shield and brake hose brackets on my 69 GTO. It seems to come down to one of these two in the photo. One has coarser threads like those used to attach sheet metal to the valance and is 7/8" in length (minus head). The other fastener, with the finer threads is very similar, if not identical to those used to hang the mufflers, 3/4" length. Here's a photo of the two competing fasteners and the splash shield and brake hose bracket areas. Thanks...