Positionning the exhaust proved difficult, but the forums (fora!!) show I'm not alone. In fact, move the exhaust toward the center and it will hit the nose gear, movi to the exterior and it will hit the cowl. After a few tries I have found an acceptable position with a gap more than 1/2".
The picture is taken from below and shows the wooden shim I used to set the gap between the exhaust and the fuselage.



The exhaust is then bolted to the engine and engine mount using two small stainless steel tubings and a small length of hose with hose clamps. The picture shows the tubing before the hose is installed.



And this is with the hose and the clamps installed.



There will be a tubing joining left and right parts horizontally and two other tubings joining each side to teh engine mount. The picture show the preparation for one side engine mount.