The Hooker Super Comp ceramic coated headers did not fit the transmission
support humps in my GM A-body, so I had to make tapered offset collectors.

Making the cones was easy. Get the right size tubing for the big end and
then
carefully mark and cut out the right size triangle from the wall of the
tube, leaving
a small section attached at the big end.

Carefully work over the tube so the small ends come together. Tack weld the
small end.

Slide the cone over a mandrel -- in my case a section of 2" pipe -- and
hammer the
gap flat. Tack the center.

Smooth over any other places that might need it, then weld up the cone.