Well the first reason I did aluminum was because it's cheap.
The second was because it's going to be a eight-year-old boy riding it.
And it notches so easy with a end mill.
No paint or powder coating required. ( fan cooled so no melting snow to stick)
And welding alum is fun!
Going to use cromoly to build the skid.
I have an IQR front end I was going to Cut narrower and use but then I wieghed it compared to trailing arms and it was 7# more.
Decided to just go the easy route and keep the bulkhead and engine cradle.
As for aluminum not being stout enough; I've built a few full mod Hillclimb sleds with 3/4" x 0.0625" wall 6061-T6 Aluminum square tube and as long as you take into account the direction and vector the forces that will be applied to it and triangulate accordingly. It will survive whatever you throw at it. I did run a small strip of thicker 1/8" plate where the Tison style M-10's mounted. And 1/8" wall rectangular tube for the rigid engine mounts. The only time I tweaked one of them (still ridable and did not notice the tweak until days later) was when CC Skidoo rebuilt the rear shock for me and put in some water contaminated oil, the shock locked in the air while jumping a catwalk at the Alyeska Hillclimb during competition. For a daily rider, I'd bump it to 0.080" wall aluminum, but I've progressed to CF wherever possible now.
Keep posting up the progress, it is always nice to see what others are doing, sometimes it gets me thinking and I change my approach to something.