You can't use an LC580 board on an LC520 without performing some major major surgery - the LC520 (and LC550/575, and Colour Classic/Colour Classic II) use a different motherboard edge connector to the LC580 (which uses the LC630/PowerPC Performa style of motherboard edge connector), the only way around this issue is to replace the LC575's internal wiring loom with one from an LC580, which requires major major work, and a fairly decent understanding of electronics.