spririlsmurf .. sorry you are dead wrong
wrx manual front 3.9, rear 3.54, final drive ratio is 3.9
the wrx tranny has a 1 to 1.1 reduction
but for example a 2000 rs tranny has a 4.11 front and 4.11 rear with a 4.11 drive ratio (no reduction gear)
as mentioned above you will need a WRX clutch and fly wheel
your rs one won't work
also switching to a wrx tranny will reduce your final drive ratio .. (read loss of effective power but better gas milage)
why would you want to do this?
edit n/m thought ou werer dooing this to a RS .. an L comes with a 3.90 front and rear ..