Not a problem. Back up right to where the box and the stinger are about to touch. Adjust the box height to where the bottom edge of the box opening is just barely below the bottom edge of the stinger. Jack UP the jacks so the angle of the box matches the angle of the stinger. NOW, because you are going uphill with the truck, stop when the stinger is about half way into the box. Readjust height, again so the bottom of the box is touching the bottom of the stinger and again use jacks to match the angles. Back the rest of the way in, and do your normal fastening and bar tension.
I CHEAT.....this is where a magnetic camera on the stinger, forward of the ramps, gives you all the education you need....and perfect hitchups every time.
"If the women don't find you handsome, they should at least find you handy." - Red Green