[Tezzating]

EJ257 will work with the XT ECU if you install XT/WRX exhaust cams and gears and get it tuned.
Your easiest option is as above, new EJ255 short block, salvage the heads/cams. I'd replace the oil pickup, oil pump & sump with new.

I did it the expensive way, EJ257 and modified STI exhaust cams with vernier gears. I dont recommend it.