Use the corner of a fine file & make a small notch. It will teach you how to tell hardness of bolts, and other steel, like tubing.
Scrap sheet bits thrown in a box, you can tell 4130 from mild steel when you need to. And you'll then learn to tell by sight(not always) too. Always check.
Grade 5 probably ok but I'd bet Attorneys made grade 8s standard for engine mounts, for liability.
Engineering safety factors are usually ten times what an actual load is for that reason.