Of all the DSE firewall installs I've seen in the various threads here and elsewhere I felt Jay's approach was one of the best. His thread can be seen
https://lateral-g.net/forums/showthread.php4?t=9421 but it looks like some of his older pics aren't showing up any longer.
I think plenty of installs are done on top of the original sheetmetal and given the rolled edge I think it would work ok. I was just concerned about the risk of noise/vibration from the gap between the panels but that could be mitigated with some seam sealer probably.
Or you just cut most of it out like Jay did and not worry about it. In addition, he drilled the spot welds and completely removed that outermost layer of sheetmetal. The added benefit there is your fender mount isn't pushed out any like it would be if you just added another layer.
I found these in his other sites
gallery
