What you need is a swan neck header. You also need to know how much the canister center line drops in relation to the engine exhaust flange center line. 0, 25, and 50mm drop are standard dimensions. other values can be achieved by inserting two ball joints (2x) (MTW calls these knuckle joints). They also are great for making that just perfect fit.

If you do not want to use the swan neck header, you can fit the muffler to the engine and then define the engine mounting angle in order to line up the parts with the tunnel.