qt 400 to s.s.

Hello to all, my question is, how do you weld qt 400 to s.s. using m.i.g.? We are building a cyclone and parts of it are qt 400 and parts of it are s.s. Would you have to preheat the qt400 or can you just start welding once it is rolled? Most of the material 3/16" and some 1/4". I have welded qt 100 before with a little preheat and using 7018 but never qt 400 and qt 400 to s.s. Any input would be greatly appreciated, thank you.................Ed