I am building some Turbo headers for a twin turbo setup and was planning on using 1/2" thick mild steel header flanges and 304 ss plumbing els. My welder advised me to get stainless header flanges. He mentioned that welding stainless to mild steel would be possible but was worried about it cracking. What do you guys think, will I be alright with my original plan?

You can get S/S filler for S/S to mild steel. Back purging or Solar flux would help make a better weld. I would not weld S/S without either of the two. If it was my setup I would use mild steel flanges. Have you asked the fab person about possible expansion problems with S/S flanges at the head?

Thanks, I passed on your comment to see what he thinks. I wanted to keep with mild steel flanges just for the heat expansion reason.

After discussing it with my welder, we are going to use the steel flanges and use 309L filler rod.

That is the way I would do it, mild steel flange and 309L filler with Solar Flux B or argon back purge.

I used 312 filler rod, seems to be holding up just fine.