That is a big gap. I probably would fill it with 45% silver.
One thing I noticed from your lug is that you are filling from the shoreline. Leave the seat tube long and fill from above the seat lug. Then pull the filler through to the other shorelines. Similarly, leave the head tube long on top and bottom and fill from there. Makes for much nicer shorelines and you can be sure you filled the lug.
The stainless looks good. It is more difficult because it's easier to overheat. Just a little bit of extra heat and the joint is cooked and it's difficult to flow silver. Looks like you have your heat control down pretty well though. In that case, it's no different than other steels.