Very good to see this aircraft taking shape. The winter camo still looks very good to me, and I'm happy you negotiated the difficulties encountered.
Exhaust tubes look great, and your technique sounds very useful and practical.
The boxy thing is, I believe, the optional intake to the carburettor for heated air (to avoid icing up under those iffy conditions near or below 0°C). The air is sucked through the two collars around the hot exhaust tubes, and thus get heated before entering the carburettor intake (boxy thingy).
Very good work, happy for you! - L.
PS.
I went with the original prop as well. Considered making a more advanced version. Tried it once, didn't turn out well. My engine (with exhausts still unmounted) is
here.