Do you mean Krypton Green and acrylic urethane?
I was a kid when I worked in a body shop so take what I say with a little seasoning.
The paint would not be original so, in my humble opinion, spray the firewall a good generic black.
That way if the green is regretted later on at least you don't have to redo the firewall.
It's difficult to keep overspray off an engine, wires, cables, hoses and other parts.
Also you need a good experienced body man to spray the truck. Solids are easier but metallic paint, especially on big rounding fenders, can run (collected metallic particles) that can be seen (the reflection) by the eye even though the surface is smooth. Maybe modern paint today doesn't have that problem. If it runs, you can try to let it dry up to maximum limit, then spray another coat. Fingers crossed it doesn't run again.
