Nothing special... just the front end cap of the rocker panel.
You'll need to keep the mounting point aligned properly, since the bottom point of the fender mounts directly to it - depending on how you affix it, an "L" bracket could prove very challenging when it's time to put the fender back on. It'll also need to be somewhat flexible, to aid with alignment of the fender.
I recommend getting a "Body Assembly Manual" for your year model - it should show proper placement and relative orientation.
Currently, there are repop pieces for the rocker panels, but my experience is they are not even close to the same shape and contours of the original metal (more like the aluminum rain gutters I recently installed on my house, if anything) - plus, they don't cover the area under the fender. Your best bet is to harvest a good piece from a parts/donor car, check with Don at OMS (he might have some pieces available), or just repair the holes by welding in some new metal.
While you have it opened up, make sure to kill as much rust as possible, and treat the inside with some rust inhibitor and a hearty coating of Rust Bullet, POR-15, or your preferred epoxy-based enamel. Taking a trick from QCode351Mach, get a sponge that just barely fits into the 'tube,' load it up with the coating, and push or pull it through to cover the inside with a stick or string, respectively. It'll go a long way to making sure you won't ever have to get back into that area again.
Hope this helps.