So what would have Ford used 20 years ago?
My opinion is the cheapest thing that would look good. Definitely NOT 300 series stainless; too expensive, besides, you said it's magnetic, so it's NOT 304. Regardless if it is 400 series stainless (which I doubt), you just embedded rust into it if you used a carbon steel wire brush on it. You use a stainless wire brush on stainless.
If not knowing what type of steel it is keeps you awake at night, but you're not willing to pay a testing laboratory to use a PMI gun on it, you can identify a lot of basic materials with spark testing using a grinding wheel, these videos aren't very long and may help satisfy your curiosity: https://www.youtube.com/watch?v=GnSBSKTC834 https://www.youtube.com/watch?v=oId-fXqk5mU
You can probably obtain different types of material at a local nonferrous scrap yard. Hit them with a grinder and you can QUICKLY tell the difference in sparks from stainless vs mild steel. IF you find that it IS stainless, which would also mean it's aftermarket, and you want it to look nice, go someplace that sells welding supplies and buy a NEW stainless wire brush, or sanding materials made for stainless, and don't use your new tools on anything before touching your exhaust; once you hit carbon steel with them, don't ever go back to using them on stainless. They're contaminated.
Hope this helps you.