Yikes! Having made many adjustments on these old doors, I would never take an impact wrench to any of it, not even locktight, don't make it any harder, you're always going to need to make adjustments. Get fresh screws with the right countersunk head. Pantera Performance has them, the ones at your local hardware store won't sit right. Then get the RIGHT sized Phillips screw driver. Tighten by hand very firmly. Also, consider it's all rusted back there, you may very well wrench something off, or strip the threads, and then God help you.