The little bit of reserch I’ve done seems to lead to using 309 filler as a good filler for welding SS to mild steel.
Stainless isn’t all that great for strength. It galls pretty easily. The shear strength is much less than normal bolts.
The shear forces aren’t that critical and I was thinking about using grade 8 instead, but being outside, I want to try the SS. It gives me an opportunity to not only offer a good solution to this project, but gives me some experience welding with stainless.
I don’t know if you feel the same way, but I am more motivated to learn how to do different thIngs by actually designing and building something useful, even if it fails and has to be fixed, than running test beads over and over. 
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If strength isn’t a factor You should be okay. But you may experience some localized rust and staining at interference of two metals. If possible, paint that area to exclude water to help and reduce galvanic action.
Good luck, it is weldable with right rod. Make sure the area being welded has the zinc coating coating removed first before welding. Not only to avoid toxic gas but better weld. If cadium plated bolt, also remove plating.
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These aren’t stainless, but they are my go to solution for nuts inset into a steel tube 1 inch or larger:
Drill a 3/4 hole, use a small magnet to hold one of these flush, tack it in, then finish welding it up. No plating to worry about, though you probably want to run a tap through it in case you heat shrink it.
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Now that just makes it tooooo easy… Glad I know about these though now.
I can see using these in other applications.
On this project, I’m going to use SS nuts and bolts to try it out.
To provide some real world experience with stainless welded to carbon, about 10 years ago, I built a number of wire frame Christmas decorations out of 1/4 round rod. I MIG welded 3 1/4-20 nuts to the base ring with er70-S6 MIG wire. I got good penetration into them, and had to run a tap through for shrinkage. They were paintd, and have 10 holiday seasons of use, and though they look rusty, they are holding up fine.
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