I can see that the cross-sill would be in the way. Mine was disassembled when I worked on it, thus my thought that you could reach it from underneath. I recall removing the two bolts thru the cross-sill and the front of the stake pocket from underneath. But the upper tailgate hinge is above the top of the cross-sill.
Again, grind the head off the bolt to remove the hinge and hope that the captive nut is just spinning and doesn't fall out. Then tack weld it in place and remove the cut off bolt, drilling it out if needed.
Removing the cross-sill likely entails grinding a weld off on the rear of the bed. I recall having to do that.
