Whatever you do, make sure you put the trunk floor and extensions as well as the rear valance before you weld anything. I did lower quarters on my 70 Challenger as well as trunk floor. I didn't fit the rear valance and was very disappointed when it came time to assemble everything. Consequently, I mocked everything up when I did my Cuda and the net result was that everything fit as it was supposed to when I was done.
In other words, I made the mistake of thinking that the parts all fit together (like a model car) and that if I aligned panels/parts as they appeared to fit that it would all work; NOT SO... there is lots of adjustment that can/must be done. Lastly, in both cars I had to trim the trunk extensions (typically the bottom).