Appreciate the information... from everyoneTony, there is actually a foam gasket available from Mike's Montes that goes behind the delete plate. I actually have two delete plates, because ya know, redundancy lol! I have an inner plate that is welded on, then has RTV along the seams. All of my electronics are mounted to it. Then I have the gasket with a small amount of RTV with the outer delete plate screwed to the firewall.
I don't want to take up any space behind the delete plate because I have no idea how much room I'm going to need for Vintage air unit. In your situation I can see that would be perfect for mounting electronics.