I think you are going to run into the same issues us Impreza RS owners have when doing a conversion to STi. Harness is so different that you are have to do a full swap or a lot of rewiring. Like staying up till 3am all week crying because nothing is working right lots. I believe you have an OBDI (that changes to OBDII in 1998) That could be an issue for your areas emissions requirements, you’ll want to do some research there.
Thinks to investigate:
• Custom brake lines for the ABS have to use the STi’s system for this.
• Rear suspension shock mounts might be too small for the new STi rear end.
• Custom peddle mounting for fly by wire
• You’ll need to use the STi Dash that will need to be custom fabricated.
• You aren’t going to be able to use the STi gas tank and evaporator system, which will toss a CEL, will need custom work.
• You’ll need to do some custom pipe work because the exhaust is longer on the Legacy.
This is me just talking out of my ass about it; all my swaps have been done on 1999 to 2001 RS from 2004-2007 donor cars.
I’d love to see it work out. Do you already have a 08 donor car picked out?