Haven't done it in a while, but I used to buy steel exhaust tube U-bends real cheap thru J.C. Whitney's catalog. Hacksawed the shapes I wanted from the U-bends and gas-welded them into exhaust or intake manifolds. With an intake manifold, you could braze instead of weld, if you feel more confident about avoiding pinhole air leaks that way. The flange(s) of the manifold should be one continuous length of steel while you are fabricating the manifold; after that you can cut it so as to have individual flanges. When you're done, get the flanges milled flat. Good cheap fun.