Have you cleaned the idle control valve yet? Also try cleaning the throttle body paying particular attention to the butterfly. I have had a few occasions of sticky throttle bodies on similar vintage Legacy / Outbacks. If you manually actuate the butterfly does it feel like it "pops" when you start moving it?
BTW how is your cooling sysem routed? Are you mounted under the bus or in the engine bay? Been contemplating an EJ swap on my 75 Westy.