well if you want to make a lot of stuff that requires back purgeing:
www.weldhugger.com
or since you are doing a header and want to di it cheaper...block off one end of the tube you are welding on and on the other end block it off but put the tube from your flowmeter through the block off. You will need a second argon tank, flowmeter, and tube to backpurge sucessfully.
I had a friend that made a "orange juice container" *cough* out of stainless and just stuck a second tube in there and blocked the rest off to keep it safe to drink out of