Which way round are the arches supposed to go as one end is different from the other i.e it has a wider bottom edge.

Any advice would be helpful.

slanted edge should be at the front to allow for the exhaust can to go past it. Lower of the slant against the side panel and higher edge of slant furthest from side panel.

Hope that makes sense!!

HTH,
Rich.

Most horizontal edge to the rear... when flange is on sidepanel.