Matchless,
I also have some super stuff I bought from a foundry supply house (where I get my casting sand and other stuff) its a German made aluminum repair formula that is unbelieveable. Its about 96% aluminum and some kind of synthetic binder. Its sold to foundries for repairing casting flaws. It will withstand 700 degree heat, and after curing machines just like aluminum (since it is basically all aluminum). I use it for making headers and fixing engine exhausts. It will take the heat and stand up. If you need help making the header/pipe junction I can also help.
FAiling that, I can allways make a wooden pattern and snad cast you a one piece header from aluminum to your specs.
I want you to succeed buddy!
Andrew