I would go a step further than this guy, I would weld the frame and the 2x6 together, grind it flat, then weld the plating on top of it. Then brace it. Everyfuckingwhere.
But I don't see much of a point having the frame rails that far apart. If you're not doing center mount equal length control arms, why cut it at all?
I would want most of the motor in the cab, tie the frame in with a horizontal bar, extend two rails out from close in the center, just in front of the motor.