Definitely understand that. You will be very happy with the end result, not sure you will be happy with what you pay to get this installed and running the way you want, lol. It is possible. I would say make sure to start with a manual Outback/Legacy GT if you can it will save you a lot of money, time and stress.
You can use the STI drivetrain, but you will need hybrid axles for the rear or you can use the legacy GT front axles or STI front axles if they came out of a car that was 5x100. For the rear I would suggest using 07-09 spec b rear axles as they will be plug and play with your r180 rear diff, otherwise you will need to create a hybrid axle using the inner axle from the STI, and mate it with an outer Legacy GT.
I did convert my 1st 5spd legacy GT into a 6spd legacy GT(from a specB so as plug and play as you can get if you have all the parts) and the increased mile/gallon was nice while I had it. It also is capable of handling more power if you go with a solid case 6spd as opposed to a split case 6spd found in WRX