I wana know how they weld up the inside of those merge collectors. They look like they are made from 4x 45degree bends that have been grafted together. I reckon there must be some tricky way they make them to get the insides so nice.
I did mine by welding up a steel pyramid shape into the merge section made from triangular sections of tube. I weld them together along their edges and then sharpen up the welded edge with a die grinder.
The pressed 3 1/2" collector tube then slips over the whole thing and gets welded in place. Not as pretty looking but a lot easier to make.
Keep us posted.
Cheers