Just reading the title of this thread made me stop dead in my tracks. Are you seriously considering doing this? Surely not.....if you have never run a gasser at all, I can see where this conceptually might seem like a reasonable thing to do to save weight. If you have run a gasser, then I think you're pulling our collective legs.....

1. Mounting bolts for ANYTHING larger than 20cc needs to be 5mm or larger, if not for any other reason than stronger thread strength in shear. No question in tensile strength.

2. There are effective methods of attaching boxes to firewalls using angle aluminum stock, bolts, etc. I'd do that before building a balsa contraption with long bolts. Building a box and attaching a new "firewall" would permit shorter bolts and standoffs, and may address some of your needs.

If you are intent on using balsa and tiny hardware, at least don't fly around anyone or other equipment until you have maybe 300 flights on it...... it was scarier to go back and reread. I think you're messin' with us.....