Why would you ever use an aluminum bolt? And don't worry about the torque in the eyelet, especially on a steel frame. Put the rack on closest to the eyelet, because that's the source of bending loads. The fenders are essentially no load. Use an internal hex head steel bolt, which will be a high grade bolt. I prefer stainless bolts where possible on a bike, but high quality plated ones are second best. Don't bother with a black-finished bolt -it will rust. BTW, I use Anti-Seize on most bike fasteners, and lock washers on rack bolts, since they are subject to vibration that can cause them to loosen. Yes, I'm an engineer.