Actually there is a kit required in addition to the 6 speed itself.
5-Speed to 6-Speed Conversion Kit
This kit includes all parts necessary to allow the use of the STi 6-speed in a standard Impreza model originally equipped with the 5-speed. Purchase either RST-1102 or RST-1102B gearbox and add this kit. Special Order 6 Weeks.
Kit includes the following parts: propshaft, STi short-throw shift linkage, standard STi 240mm flywheel, flywheel cover, standard STi 240mm clutch assembly, transmission crossmembers, STi hardened transmission mount, clutch fork, exhaust hanger, STi standard shift knob, shifter trim & boot, rear ring & pinion set, rear diff. gasket & seals. Different clutch & flywheels can be supplied at additional cost.
RST-1102kit 5MT to 6MT Conversion Kit -- $2300