Hey Travis I'm a Journeyman Millwright. I've been welding since I was 12 or so. TIG welding is second nature to me
trust me I'd make it out of alum probably 6061 or 7075. Whatever I can get my hands on. The one thing I would strongly suggested to you about these manifolds. When you make the flanges to bolt them to the LIM etc make them thicker so you can mill them flat after welding
If you get carried with the TIG on aluminum and not pay attention you'll warp the hell out of it. If you're very careful or have a jig you'll get minimal warpage.
