Reply to "Steel rear quarter reinforcement"

If you look inside the engine bay at that point, you'll notice that there is a panel-overlap on the inner rear fender clear up to where the the weatherstrip fits, and the whole area may have lead or factory bondo. This stuff needs to be completely cleaned out befor trying to fix the area. The non-rigid panel overlap is IMHO why we get cracking there (on both sides).
If you're going to the trouble of welding a doubler outside, I suggest also seam-welding the entire inner panel all the way down to the rear subframes to stiffen up the rear section. Seam-welding all the rear panels where there are spot-welded overlap panels (like under the tail light buckets) greatly stiffens the rather flexy Pantera in the rear and prevents further body-cracking with big sticky tires. Finally, replace the stock (and useless) bay brace with a properly designed unit that jams into the holding pockets, rather than trying to hold cornering loads thru two flimsy metal tabs (with slotted holes) and bolts.