I did a little more digging around and learned that 409 ss will be magnetic and 304 ss will not. Even though i read that the Magnaflow muffler is 304, a magnet did stick to it. All surfaces including the pipe extending out were magnetic, so that leads me to believe it is 409 ss. In that case i will just mig it as usual like i do with the dynomax ultraflo welded muffs that are 409. Never had a problem nor fatigue/ corrosion issues at all.
Thanks for the feedback.