I would try to get hardened steel rather than standard hardware grade.

Lowes web site only lists one M7 nut. At 68 cents for 5 nuts they are probably not hardened steel. The store hopefully has a better selection. Metric high strength is called Class 10 Alloy Steel.

They may or may not have the 5/16 nuts in Grade 8 or Grade C. According to McMaster-Carr, grade 8 and C are equivalent.

If Lowes doesn't have what you need, try an auto parts store. Ask a counter guy because they often have bins in the back and the selection/price is usually a lot better than the little packages hanging on the racks.