A few tips:
-Close the box sections for the frame, it'll make them significantly stronger.
-Add in a bellypan, it'll help with torsional stiffness
-The split front part isn't very rigid either - extra support there (+bellypan) can help with that.
Check out some of
971's drivetrains for some good methods of sheet metal implementation. There are some helpful rectangular and octagonal drivetrains in the 2015 section that you should definitely check out.