All you have to do is cut and grind off the existing door hinge. You do not need to get behind the hinge.
Cut and grind the old one off ... grind down excess hinge on the cab panel ... then weld on the new one to the existing cab panel. I suggest that you have a continuous weld all around the door hinge. That is what I do .. but a some stitch welds are OK. Do not just tack weld the hinge to the panel, or it will break.