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Edelbrock Perf RPM head requires a 14mm x 3/4" reach plug. XR4 & 5 are short reach plugs. 4 is hotter than 5. 10.5 compression with 600 hp should probably be in a 5 or 6 heat range depending on fuel type and quality. Maybe colder yet? Some NGK numbers to look at would be FR, or GR series...