Brass is usually a good choice, however since they are a softer metal they don't grip the block as hard as steel plugs & have been known to pop out occasionally, usually when the engine is spinning big RPM's... High RPM's means the water pump is putting more pressure on there cooling system, it also means the tires are borderline to loosing traction..... Dump four gallons of water if front of the tires & life can get pretty exciting.....
In the past guys would drill & tap holes & use retainer straps over the plugs, or There are brass/copper expanding plugs that lock in very securely...