Where did you get that manifold/header from? It looks nice. I've had a look at BMB, MTW, Krumscheid systems, there doesn't ssem to be a nice curved pipe like Dick's. Is the length of the header critical like small 2 stroke engines with tuned pipes? The total length is a problem, the wing tube is in the way. The total distance from the engine exhaust port to the tube is 400mm.
Here's a photo before I cut anything, Ive moved the rudder servo closer to the tail. There's a shroud around the exhaust and the fuel tank is above it.

Kevin