We have successfully installed an LS7 clutch assembly into a 2005 GTO and a 2004 GTO.  The trick is you have to use the Y-Body style LS2 flywheel, it will not work with a GTO specific flywheel. 

Weight Info:
Stock LS1 (2004 GTO) - 49lbs
Stock LS7 (2006 Z06) - 57lbs

P/n 12571611, Flywheel, LS2/LS7:
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Suggested List - 272.99


P/n 24236500, (NEW PART # 24237690) Press Plt/disc, LS7: (NEW PART #)
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Suggested List - 457.86

(Part # for Pressure Plate Bolts - GM 11516862)

LS2 GTO Flywheel


LS2/LS7 Corvette Flywheel


Another pic of LS2/LS7 Corvette Flywheel





LS2 GTO Disc


LS7 Corvette Disc




LS2 GTO Pressure Plate


LS7 Corvette Pressure Plate


Another pic of LS7 Corvette Pressure Plate