I need to replace my gear box. I removed the old one and bought a used one with the pitman arm still attached. My question is how do I know if it's centered so that when I install it my steering wheel will be straight, wheels straight, and I have the full turning radius from right to left?

 

You have to install it in the truck. Make sure the wheels are pointed straight and adjust the pitman arm so it lines up with the drag link and then hook up the steering column and make sure the steering wheel is straight and that should do it.