Managed to fix up the other rotted out flange yesterday. Using the same process as the other one - cut off the rusted out existing flange, clean up for welding, cut a new steel ring, weld in place, grind flat. Then I mounted the header to see what needs to be done to join the header to the silencer in the correct position - a nice up-swept angle and slightly out to the side. I will fabricate a kinked tube to match the required angles and then weld it in place.