[Gustav]

What material was the 420 grams for?

It looks like it would be extremely beefy in 7075,maybe a little over engineered if you see what i mean,unless you're wanting it to last forever,it probably would in the the 7075-t6.
2.5mm 6061 might be enough for the upper plates if you wanted to keep those sacrificial,before they started to bend screws and knacker the threads in the chassis rails.

it's like a viscous cycle though in terms of weight.The stronger and heavier you make those upper plates and the skids,the stronger and heavier the rails need to be,if you kept the thickness of those down then you might be able to lighten up the rails more since the the plates would be the sacrificial part.crumple zones:005: I would want it lighter since you're going to all this effort to make an efficient design,otherwise you're probably overengineering in terms of the strength it needs.A g-maxx comes in at like 230 grams doesn't it? it's something like that.