Smooth firewall question

[FrankenChevy]

Thanks for the feedback. I just filled them all this morning so now it's just a matter of putting new holes back where I need them. After all of this, the firewall is not as flat as I would've hoped after filling the holes. This is primarily due to panels not aligning. I was able to keep warping due to welding down by working slowly and letting it cool but I couldn't get everything to line up in a plane. I knew this was a problem before filling the holes. I've replaced the complete firewall area (upper and outer cowls, firewall, and upper dash) with aftermarket parts so I did the best I could (i.e. pulled, tugged, beat, etc) but the panels did not fit properly (or the 40 year old car was off). I even had to split the new firewall down the middle and take out a 1/4" wedge to get it to fit since the center to center distance on the subframe mounts did not match-up. I guess I'll have to go with a second plate welded on the firewall unless the future bodyman can make this work (without tons of filler). I've read the posts on this subject and it seems that putting filler on this thin of metal can lead to cracking (definitely makes sense). I was wondering if this secondary panel might be a good candidate for bonding on using structural adhesives (after I hammer it more flat than it is currently)? Not sure how I would address the edge treatment (transition to the adjacent panels) but the center could be held with these adhesives. Any thoughts on this idea or has anyone done something like this? I've even thought of bonding on a thin, flat piece of carbon fiber in this area. Not sure if this will look good or not. I'm looking for the resto-mod/PT look without venturing into the import-look. I also have the same problem with the edge treatment on a bonded CF panel.