I am going to assume you have a chromemoly steel of no more than 1.5% chrome and 0.5% moly.
Preheat to 100 - 150 Deg F. Use filler wire to ER80X-B2L Or for this small job if you have 309 or 310 stainless that filler could be used. Argon gas is OK.
For these thin sections as welded joints should be OK without post heat treatment.
Joint design -Fillet welds should only be used in welds of low stress! Butt joins are preferred and rapid changes of section should be avoided. Ensure full penetration.
Source Welding Habndbook AWS, 7th Edition Vol 4.