instructions say to us a 5/16" drill bit yet the hole is abut 1/2"/

You are going to want to use carriage bolts to mount these brackets to the lumber. In order for the square collar on the carriage bolt to seat in the lumber to self-hold, you are going to need a half-inch hole in the bracket. Center the 5/16" drill hole in the 1/2" bracket hole, and then drill the hole. It will work out fine. The carriage screw head will sit on the rim of the half-inch hole, and you will be in business. The alternative to this arrangement would have been for Reese to broach (form a square hole) in the bracket, which would have increased the cost considerably. You will just have to trust the instructions on this. If you don't mind a protrusion in the finished assembly, drill a half-inch hole in your lumber and bolt the bracket and lumber together with 1/2-inch (3/4" wrench) hex cap screws and go that way, but you will have the bolt head of the cap screw sticking up, and that might make whatever you are rolling up the ramp get stuck and/or be harder to load. The carriage bolt solves that problem to a degree, because the head is rounded. Good luck.

5/16 In. bit is the correct bit to drill the bolt holes.