one thing that I found to work well for spot welds was to look in the factory assembly manual for their locations and how many there are in the area, then drill them right through with a regular drill bit and then I have a heavy duty scraper/putty knif that I sharpened on the side edge. i use that scraper forced in between the panels and then hammer it along the seam to cut through the rest of the spot weld that remains after the drill through process. this leaves a hole where every spot weld was, I know, but then I use that hole to re-spot weld the new panel on. I got tired of buying spot weld cutters for the same reason, but there is a spot weld cutter that looks like a drill bit andseem to last longer. the link shows a pic, mine were a cheap set and lasted not bad before the nub on the tip would break off. I did sharpen a few with success.
https://www.autobodytoolmart.com/pro...neumatic-tools
I tried this style but found they break so easily. maybe because my panelled areas were not very flat when I tried this style
https://www.tractorsupply.com/tsc/pr...e-cutter-17224
you could step flange the edge of your firewall panel to fit behind the lip and still be able to spot weld it in from the front side. I have a cheap air operated step flange/punch tool that works pretty well but it doesn't have a very deep lip on the flange. I am sure there are others available, even some vice grip style ones, or you could make your own jaws for a vice grip. or if you know somebody with a bead roller they could flange it for you
https://www.ebay.com/itm/Air-Punch-F...edirect=mobile
https://www.ebay.com/itm/Blair-Sheet...edirect=mobile
I have one of these air hammer flangers as well but find it is hard to control unless you have the panel secured well. the tool tends to want to go it's own way so you need to force it against the edge of the panel as you work across the edge.