I would drill the torx bolt out until the head pop's off. From there, you may be able to tack weld the plate and remove it with vice grips. Or, you may need to drill it all out and install either a welded in nut, or a riv-nut style retainer.
PS, I LOVE the look of the CJ gate on my YJ. Here is a trick for the latches. You can tack weld the old D35 u-joint straps to the tub once they have been tapped down enough to allow the latches to slip over them properly. (If you have not converted the straps to u-bolts yet, it's the perfect time.
PS, is it the hinges on the old CJ tailgate, or the ones on your Jeep?