I've done a fair amount of carbon to stainless, primarily 304, but also a bit of 316l, in precipitators and associated assemblies. Carbon to 304 is easy enough with 309, and doable with 308 as well, but remember that the filler doesn't want to wet-out well into the carbon, so be diligent in making sure you have good fusion. It can be coerced with a smooth hand. And once done, it looks pretty cool, too...especially to the guys who aren't sure just what you did.

The carbon to 316 was just a PITA, but again, due diligence will pay off...as will a boatload of practice.