Reese Universal Base Rails - Questions on installation

I have a hijacker I am fighting to get in now, it is the slider style and rails run the length of the truck bed, and on mine it had to be assembled then put in box and back of rail was set at 24 1/2" from end of box, then I squared the hitch side to side, so if yours is assembled when you put the hitch in and set the back rail correctly then the front rail has to be the correct distance.
You would then just make sure side to side is correct.