A steel frame isn't the best conductor of electricity, although it will work. I ran my cables through the floor (grommet w/ silicone sealer) and along the frame under the car. They're held in place with rubber-insulated clips every couple of feet.

The positive goes directly to the starter.

I ran my ground to a stud on the frame, then from the frame stud to the engine block. I also installed a ground jumper from a transmission bolt to the frame, and from the engine block to the frame. Redundant, maybe, but a good ground is an absolute necessity.