Spark Plugs for LA w/magnum heads

FR4 in NGK is a .750" reach, 5/8" hex resistor spark plug for mild compression/driver setup, 4 being a hotter plug.
R5671A-7 is the race style plug that is the same .750" reach gasket plug with 5/8" hex but a 7 heat range, being a colder plug for higher CR.

General info above, heat range would depend on your actual combo and use.