I've done a search, but as everyone knows, the search function kinda blows right now.

I have a someone who does good welding available and I want to make up my own manifold. I have all the flanges in mild steel; exhaust, T3 inlet, and wastegate. I need to know what piping I should use. I'm planning on using 1 1/2" mild steel, but what gauge should I use? I don't know that much about steel, is there specific grades of mild steel, like stainless has? Which grade should I get as well? My friend will be ordering it all and we'll be tacking together a prototype this Friday.