I'm in the same situation, the best way would be to remove the headers and get the 12mm narrrow band bung mad into a 18mm wideband, but pulling the headers is a pain in the arse.
So, i intend to get the 18mm bung i have, welded about halfway between points B & C on your picture.
So, i intend to get the 18mm bung i have, welded about halfway between points B & C on your picture.