I've been biting my tongue for days, to avoid being negative.
Please do, its much more enjoyable when you do.
So I will try and be as positive as possible.
I see the tube chassis and Carbon Fibre skins as needless redundancy. With the stock cooling extrusions and Carbon fibre skins and doubler plates instead of the aluminum, you are still lighter than stock and far stronger. A 4'x8' sheet of 1/16" dragon plate is less than a grand and is plenty to do the full tunnel, doubling just the known crumple zones or the whole thing, if you feel the need.
I am a welder by trade but would never put extra steel into a snowmobile. I try to eliminate all the steel in my sleds in favor of lighter, stronger materials (i.e. Titanium, Aluminum, Carbon Fibre, engineering plastics, etc.). I get flamed every time I use the cliche but I see steel as "Mid-west farmer technology".
You will not reach your projected weight target with that approach, you may get sub 500#'s dry but not 420-430 like you are hoping for. I hope you prove me wrong, and regardless it should be a fun sled. Build on, we will be watching.