[Tireiron]

Only the USDM 06 and 07 STi 6MT have a 1.1:1 center differential and use a 3.54 rear differential. That is the only 6MT that would allow you to retain the factory r160 rear diff without changing the gearing in the r160.

The r180 requires such minimal modifications to fit that it is totally worth doing. You will use the STi rear axleshafts with it, which also makes replacing shafts in the future easier than sourcing the correct rear SVX shafts.

You can use the front SVX shafts by adding the stub shafts and seals into the 6MT, although some of the older 6MT already had the shafts in them. Adding the shafts is incredibly easy so that is not a worry.

I used the SVX driveshaft, but I have heard the STi shaft is the same length and works for the swap, best to check the length and confirm. It certainly will make sourcing a driveshaft easier if the STi one is the same length.

Some of the older 6MT are non-DCCD and may be a better option if you do not plan to add a DCCD controller. Although a 6MT with DCCD will work perfectly fine without a controller, you will not get the full benefit of the center diff without it.

As mentioned you will need a clutch and brake pedal set. I used a set from a WRX/STi (mine was an '05 specifically but that whole body range has the same ones). I also swapped the brake booster in from the WRX/STi which made the linkages bolt right up. You will have to cut the firewall for access to mount the clutch master cylinder and will need the line to the slave cylinder. It is a tight fit under there, but after pulling the auto tranny wiring harness back into the car there is a little room at the firewall. I had to modify the SVX gas pedal a small amount just to get it to clear the clutch rod, it just had to be shortened a small amount.