Take this for what it is but I'd try to use just about as small a filler wire as you can find. 1/16 in. (1.6mm) might even be slightly larger than ideal. If you can find .045" (3/64" or 1.2mm) that should work out well. I'd make sure everything is very clean and avoid using abrasives if you can help it, and try not to thin the edges. I'd also use a tapered tungsten and higher ac frequency and no more than 60% EN balance. A 1/16" (1.6mm) electrode is probably a good size and a #4 cup with pretty low flow rate could help with stability. I dont know what tungsten you use and this is completely arguable and a matter of preference with very minimal significance, but prefer to use zirconiated tungsten for all AC welding. I cant prove or explain this, but in my experience, it resists contamination, holds up at high amperage, has a consistent and stable arc, doesnt need to be sharpened nearly as often as any other type, and doesnt mis-shape. The only possible disadvantage is that you wont want to use it on DC at all, so it has really one singular purpose, but it does that one thing very well. I know that other people will disagree but that is my opinion on it. It wont make a big difference one way or another, the main thing is that just it saves some time in grinding and is a little more cooperative in forming a favorable shape.