From front to back.
You need to run a pull type clutch and matching flywheel of the same year. Example purchase clutch/flywheel setup from '02-'05 WRX. You need to change out your slave cylinder to the pull type one i.e. '02-'05 WRX. This is a phase 1 transmission, it will not use all of your transmission bolts and will leave you with 4 extra bolts. For the stud on the starter, use a nut of the same thread pitch as the tranny to engine bolts if it didn't already come with a starter and hardware. You need early '02-'03? front axles because this transmission uses female axles. You may need to swap out your reverse and neutral switch sensors so you can put your tranny harness onto this transmission. You may also need swap your speed sensor over. Replace the shifter U-joint with your WRX one so you can use your shift linkage and don't forget to purchase a pull spring unless it comes with one so you'll know the shift lever is in neutral. Find a 4.111 R160 from a Subaru. If you must keep it "WRX" something from a '02-05 WRX auto will have a VLSD and 4.111FD. It can even come from Forester NA 5spds, & 5spd 2.5RSs (excluding '05 2.5RS; '05 2.5RS Sport with 4.111). Your rear axles will fit right in.
So with all this said is it cost effective for you?