scratchbuilt exhaust headers

I'm interested in making some exhaust headers out of aluminium tubing. I understant I can mount it on a drill and polish the tip with sandpaper to make it sharper.

However, how would I do this if the header tips are not flat but oblique?

I assume that you will start with round tube and then gently squeeze it flat? So therefore you could thin out the wall before making it oblique.

P.S. Do you want to sell your 250 GTO ?, I just need the engine.....:D