pic: 6WD Sheetmetal Chassis

[IanW]

It looks really good.
I'd say the lightening pattern is rather conservative, so if you're pressed for weight, you could probably take some more out.
Looks like you might have more rivets than you really need on the gearboxes, but better to have too many than too few I suppose.
This may be something you intentionally didn't add, but you may want to consider a bellypan for reasons ranging from enhanced rigidity to convenient location for electronics.
My only other thought was that you might want to add more overlapping/connecting material at the corners. I have modeled a couple drivetrains like this, but have never built one of them, so I will defer to someone with more experience on this aspect.