A friend of mine had similar issues in a restoration of a "good metal" government vehicle from the war era. It had been repaired with some of the metal you're apparently working with and it welded like visqueen. I'm speaking second hand here, I didn't work on it any, but they had sections of the metal tested for its composition to get a better idea of how to work with it/weld it. I'm not positive, but I think it was chemically tested.

I've enjoyed looking at your project photos; no, you're not a beginning welder for sure!

Brad

Wrench Fetcher, PhD