Thank you all for your comments and suggestions. Right now I am caught between doubling the firewall with a single piece of 1/4" plywood and then add 1/4" to 5/16" hardwood spacers or just cut 1/2" to 9/16" hardwood spacers. I like the idea of the extra 1/4" plywood on the firewall better, but the firewall was already fiberglassed and that makes it a little more complicated. Yes Tande, 3 1/4" sounds like a lot but consider I am only adding about 1/2".