Forgive me - but this sounds like an exercise in frustration fixing to happen...lol  nahh just maybe some teeth gritting.

Are you sure they are SS?...They could easily be magnetized being SS tho....the 300 series of SS is the one the displays almost no magnetic reaction / permeability....however if its been drawn formed and cold worked extensively (like trim pieces) it certainly could have some magnetism especially considering its age.  Other grades of SS display magnetic reaction and permeability from the get go.

Maybe somone here knows exactly what grade they are ....without knowing that I would recommend a 308 filler because it covers a lot of ground on SS alloys.  I do not know what thickness you are working with here but by the sound of your post you are preparing to go to war on lace panties.  So an .032 filler is probably too big for this... .025 maybe as well....you have the option of getting a small spool of .020 mig wire which would be much more cost effective then the tiny tig rods. 

Back purge is what you should do if its thick enough to let you weld it without backing...if not try to fashion up some copper to use for backing if you have a good snug fit (clamp it together) this will keep sugaring of the backside to almost nothing.  I am not experienced with the dynasty so all I can say there is start at the bottom on DCEN (dc negative)... set the machine where you can hold an arc on this stuff with no wetting and start wetting at about 1/3 pedal and you should be about right for heat.  IF the material is less then .015 thick then use your .020 tungstens....if its .020 plus go with a sharp .040 tungsten.  15cfh with #6 cup should be fine  if you need a smaller cup to "get in there" try a #4 at 10-12 cfh.  You will probably have some issues with drawing especially if your doing a build up so figuring out a way to clamp/fixture this stuff to hold it in place will be very important.  

Good luck with this
Tommy