I did some more of these, on thinner (1/8" thick) material, and the same size studs. It worked OK, using the same machine settings and technique, and just using pure argon instead of Ar/He mix instead, and perhaps holding the arc a bit longer on the second pass to get it to "melt in" better. I was a bit concerned about over penetrating into the 1/8" material with the Helium mix (and He is pricey, so not worth using unless its really needed.)

Although the puddle didn't penetrate all the way through or anything, the 1/8" plate did deform ever so slightly; "rippling" the outline of the stud interface on the back side (which is the side that will actually show when these plates are "mounted".) It rippled from the relatively thinner material not being rigid enough to withstand the cooling-related contraction forces. It does show I got some decent penetration though, I think. Weld should be strong.