Re: Working with copper
I have to agree that copper would be a bad choice for a mechanical part like a flange. Even with some work hardening it would distort badly as the gasket compresses.
I would use some 3/16" or 1/4" brass for the flanges as someone else already stated. Somehow I missed the intention to make flanges from the tube, I was picturing an oval section for an intake port match.
You can also solder copper to steel, if you happen to have a steel flange that matches your engine or carburetor.