+1 on the massaging that will be needed for the new panel in that area. I just replaced those areas and part of the quarters on both sides and had to drill out half of the spot welds on the panel for proper fitment. Once I got good fitment, I clamped them in place and will be finishing the new spot welds this weekend. As far as the rear cross member, I had to replace mine as well. Got the new one from NPD and it took just a little bit of grinding the ends to fit properly. If you change it out, it would be a good time to check all of the measurements of the rails and so forth while down there. I checked mine because the rear had been hit, everything was rusted out and I want to be sure. The measurements are posted in a few threads here. Good luck.
Tom
As I'll be busy on this in a not too far future (just received my tail panel and my floor pan)
If you have picts of the massage... please
Also, I could locate the spot welds of the rusty tail panel on both sides (to lower quarter), but haven't located yet where they are on the lower part of the panel.
If you have some hints about where they are (how many), its a moon landscape on mine and hard to "spot" them...
Fabrice, when I took my tail panel off it was mostly rust, so I didn't really take any good pics. Almost all of the lower portion of the quarter panel that butted up to the trunk drop offs were either completely rusted or dented up really bad. I cut a lot of the tail panel out and the just ground down along where the tail panel was welded and used an air chisel and walked it back and forth between the sheet metal to find the spot welds. Then I used a spot weld removal drill bit and drilled them out to remove what was left of the tail panel.
For the massaging of the new panel, I drilled some of the spot welds that connect the section that goes between the quarter panel and the trunk drop offs so that it would flex more and that the quarter would go into place easier. On the new quarter panels, the spot welds are pretty easy to see, I think I drilled out 3 or 4 spot welds at the most on the new panel.
The bumper brackets, if you need them were available at Mustangs Unlimited for about $15-20 apiece IIRC. If you are doing the trunk floor, there was a lot of seam sealer around my inner wheel house, so make sure that you grind down there real good. I cut out the trunk floor just forward of the oval access holes and then cut/ground out the spot welds to remove the rear cross member as mine was smashed up pretty bad. I did use sheet metal screws to assist with holding them in place till I welded. Good luck.
Tom