My thought process on it:
Remove the latch from the hatch bottom, and the struts from the hatch part. Close the hatch and install hinges to the bottom. Open the glass and tack weld the upper part of the hatch in place onto the body. Cut the lower part of the hatch just above where the window seal turns up on either side. At this point the lower gate should be able to swing down on the hinges, and the upper part be rigid with the body. The glass should also still be able to latch on the lower tail gate.
From there just finish up the welds, and make some sort of latch to keep the lower half from floping open.