I ran mine on the outside of the frame rail (the exception being in the vicinity of the rear wheels, where it runs on the inside), securing it every foot or so with an insulated metal clamp....I used heater hose (split to slip over the cable) to protect it where the chance of abrasion or contact with a sharp edge existed...
I recommend that you not run the main cable inside the passenger compartment or in proximity to the drivetrain's moving parts...
I put my batt. and starter sol. in the trunk. That way the fat cable going to the front
(now to the starter) is only hot while the starter is getting power. I put mine in the
flexible metal conduit used by house electricians and it worked great. I ran it through
the trunk, down to the sill plate, up to the kick panel and through a hole in the floor
right to the starter. Really cleans up the engine compartment.
I ran my cable through the wheelwell and along the passenger side rocker panel mimicking the routing of the trunk wiring harness that goes along the driver's side rocker panel. I ran it outside of the car through the kick panel vents, and through existing holes, to the engine compartment. I use blue heater hose (my car was blue, so I thought I'd keep the "Ford Blue" theme going.) slipped over the cable before the terminal was crimped to protect from abrasion and grommets where the wire passes through bulkheads. A trunk mounted solenoid is a good idea as the main cable is only "hot" during starting, but I haven't done that yet.