Sorry, in this situation you will probably have to make a template out of card board and cut the pieces out of sheet metal, that's what I had to do in the area of the battery compartment. I found the sheet metal (galvanized) and the proper thickness at Lowes of all places. Hope this help!
This exact problem happen to me on my 78 280z one time, and I found it was in the wiring harness at the firewall under the battery box where acid had melted the wire insulation. Hope this helps!