Here is my idea to deal with a wider rack. use brackets made out of 2" x 2" HREW that are longer to extend the chassis width. 3 1/2" Tall brackets.
Rescued attachment Chassis Mods.JPG

Bracket Picture.
Rescued attachment Chassis Mods2.JPG

Would this be used in conjunction with shorter wishbones (which will change suspension geometry), or would you be using standard wishbones and having a 7" wider track (which would be a very wide locost).

I would also want to make sure that the extra torsional stress that will be applied to the frame tubes is going to be ok.