Go to Burns, and some of the other stainless header people, maybe some you tubes from guys who do a bunch of it. See what they are doing.

304 stainless should use 307L wire for most of the fab work. Small stuff. I bought a roll of .035 MIG 307 wire and use that, Same stuff as TIG wire. Tubes need to be back purged. If the joints are kept tight, you can actually fuse it using little filler.

SiBR is only used for the outside of the flange to tube joint. The silicon bronze is less tensile than the stainless. It's "softer" and has some give. It helps to keep the headers from cracking there while supporting them. It also has a lower melting point so you don't have to make the header material "wet" or melt it. It's more of a brazing process. Making the steel melt is way too hot for the silicon. And the outside doesn't need to be completely welded. The seal is the inside weld at the end of the tube. The silicone outside is a support/reinforcement.

Material is expensive. I would get some cheaper straight pieces and practice. Cutting, making perfect joints, setting up your back purge rig and your torch, and welding joints. It's a different animal than mild steel.