Looks Great C.P!
I like your method of fibreglassing things- I`ll have to give it a go!
I asked either MACS or SOUNDMASTER once about the skill required to weld the thin tubing to the thick flange, and they made it sound like it was a fine art, and I believe it..
If you have the right flux and rod it brazing aluminium is not that hard, especially if you are using 1mm wall tube or thicker. A couple of things I have discovered are that the alloy you braze can make a very big difference- 6000 series alloys I`ve tried braze very easily compared to anything else, even easier than pure aluminium, with the bonus that they will age harden and recover most of their origional strenth. (pure aluminum won`t to any great degree)
I think a big thing to get right is the flame size of the torch- aluminium "soaks" up a lot of energy so it takes a long time (well, relatively speaking) to get to temp, so I think a big mistake many people make is to use to big a flame and then find that they overshoot the heatrange and find it hard not to melt things. without timing anything, for small parts if your flame is about right it should take something like a minute to come up to brazing temp, for a 15 size heder that might be 3mins?
Anyway, you could cheat by using your lathe to make up a tube with the last 1/8 or mor being thicker (2mm wall) or better still telescope some tube together.
If on the otherhand you are feeling lazy, I`ll braze up a heder for you,
just end me a bit of paper with a pencil print of the exhaust flange area on it.
Either way, if you can make planes from balsa, you can braze aluminum!
Stefan