ORIGINAL: Planeclothes
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".
Curious, why not just get a set of new stand offs that are the correct length? McMaster Carr sells aluminum hex stand offs up to 6" long. They are inexpensive and come in various thread sizes including metric. In the end, i'd bet this is your best, lightest, simplest approach