I know how you could make them.
If you could buy the tube size(and that's the killer)you only need a die with a roll on it.
You heat the tube end push into the die and it will roll it.
Any marks you clean up with wet and dry and repolish.
I have made a similar thing in alloy.
I can draw you a pic of the die if you come up empty.
The stacks in your pic have been made that way because the stack is parallel on the sides.
meaning it started out as a tube.
Look up my post "Air Intake" that I posted on June 23, 2012 under my username. It tells you the name of the guy who has the molds for the original stacks used on the Holman Moody cars...pictures included. Last I heard, he is still making them.