Even with welding cable, if you going to run it under car, put it in some split conduit for added protection and secure it well. (experience from working on trucks many years ago)
Run the cable from -ve terminal to chassis, and use the same size cable and put a link from starter motor bolt to chassis in engine bay.
+ve run direct to starter motor solenoid. If factory fuse box is still in engine bay, run abit of 10-16mm2 cable from starter motor +ve terminal to main fuse to get your power for rest of vehicle.
you M U S T put some form of isolation device, close to the battery. You can buy truck isolators that are 2pole (which means they disconnect both +ve and -ve(earth) or motorsport type ones with remote operating.