I have an exhausting system replacing the factory cat with high flow cat... the flanges have sleeves, it slips right over the cutted downpipe where the cat used to attached to... it has a little rust, it is steel / aluminuzed steel ? the sleeve over is stainless.... since it fitted over with a little gap. between old a little rust and new pipe. what is the best process , recommendations ?

I have acess: Mig with carbon steel / flux or gassed. or Tig dynasty 200DX with 309L 308L or carbon wires , or silicon rods ?