If you weld a pipe to a flange it is mandatory to weld the pipe on the
engine side of the flange only. Then on the outside add only a few (4) spot
welds or small triangles 1/2" x 1/2".
The reason is simple, the weld weakens the metal, a spot weld distributes
the load better to full strength metal, remember it is usually not the weld
that breaks but the original pipe right next to it breaks.

Also if possible keep in mind that the exhaust will break eventually. Keep
fuel lines and oil lines out of the way of the potential leaking exhaust
gases and if you have a plastic spar like I do, do something to keep the
wing from melting off.

Marko