Box channels are the features of the floor underneath the car, where the floor pan is folded to create features that look kind of like frame rails, to give it more strength than a flat pan would have.
They are about 6-8" inboard of the rocker panel rails where you'd put the little jack the car comes with under...just look under there, farther in towards the center of the car, you'll see what I'm talking about.
In this picture you can see the red "rails" running from just behind the front suspension towards the rear of the car, ending about halfway back...they are just outboard of the exhaust system. Anywhere near the front of them is a pretty good jack stand point.
