RE: Need 3 inch aluminum spacers - help
10-32 all thread will make bolts of any length. Goop up the end the goes in the engine with thread locker. 10-32 is a hair smaller than the 5mm that the hole is likely threaded. Big dowels are good they give a large base and resist tipping better than a smaller diameter standoff. This is all well and good but if you are making 3" stand offs consider making a plywood box. It would be stronger. Then use you long bolts/all thread to secure the assembly to the original firewall.