This is the start of a CAD model which is based on my prototype collector, and this drawing is my way of working out what angle & length mandrel bends I need to fabricate a 4-2-1 collector to replace the catalytic converter. The front pipe merge collectors are well sorted, just got to do a little more work on two of the three remaining bends.

Looks ever more promising, as I have found a mandrel bender near me too.