Now finally! To the firewall.
I started with some 99 cent store construction paper and cut out a template for the massive whole in the cab.
Then, I used the template to help guide cutting out some 16 gauge cold rolled sheet. I left it a little large to allow for trimming later.
I thought a jig saw would make quick work of 16 gauge, but it just made a ton of noise and threw metal everywhere. I ended up using electric shears on the straight and shallow curved parts and grinders for the rest.
I left that lip in the firewall area from the original structure, so I just used that and Cleco'ed the new sheet ON TOP of that lip for further fitting and trimming.
Before committing to welding it in there, I did one last test fit for clearance. back onto the frame it all went. Looks like I have plenty of room.
I have some 1/2 inch square tube held to the back of the sheet to help keep it's shape. I think I will spot weld some behind the firewall, up under the dashboard for further support later. As suggested before by Dsraven.