simply take a scribe and mark all around where your weld on hinge is now on hood. You already have 2 holes in the hinge they just have some weld in them and need drilled out. The 5/16" nuts get welded to the flat stock with holes already in them same pattern as the hinge, so the bolts will go thru your hinge then flat stock into the nut. The 5/16" is the size of the nut to use, not the hole to drill. I usually will drill a slightly bigger hole to do something like this so you have room around the hole to engage the thread in the nut easily.
i added the better description to help you on the first post.