if you use the s202 rear diff, then you'll need the driveshafts and hubs too. As a part of a R180 conversion.

Otherwise find yourself a R160 3.9 Diff and its pretty much all bolt in.

You dont have to get a clutch kit. But its a good idea.

apart from the list you have up there, you will also need:

Shifter linkages and stick
6 speed gearknob
6 speed gearbox mount
6 speed clutch fork - must use this regardless of whether you re-use your current clutch or not.
4.7L of oil.

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RS Liberty - Because WRX's are only good for parts...