Nah, you don't use cones for hydroforming. I mean I guess you could but the way iv seen it..
You weld two flat sheets togethor in the general shape of your exhaust, curves and all, then just inflate it to be round, Everything strechs.
Euro Spares - FRAME ARTICLE #8 Heres a page that talks about it and has some cool before/after pictures
Note the section about having an inlet and outlet on the exhaust, for letting air out of the pipe, CRITICAL!!!! if a chamber full of compressed air ruptures, its INSANELY DANGERIOUS.
If a chamber full of uncompressable water at high pressure ruptures, Its usally just an annoying leak. But hey, Being all washed up is better then being dead.