Bumper 68 GTX

As I try to bring back my GTX, I use the empty holes as a clue that I'm missing something still. I have holes in the back of the bumper and in the front of the lower radiator support. I picked up a rock shield and fit it on, that filled most of the bumper holes. What bolts should be used? I used 1/4 hex bolts but I'm guessing they should be round top and not hex. Also, What is the hole at the end for? Rusting in my photo. And does anyone make the radiator air shield? I assume that will use all the empty holes in the lower radiator support.

Thanks

Nothing goes in that rusting hole. And yes 1/4 hex bolts are right for the splash shield. With spinning washer nuts.

Thanks, I used a washer, lock ring, and nut but I have some spinning washers that I can swap in. I should get that MMC Detroit fastener manual.