might have some luck hitting up junk yard for some older model automatic impreza and leggy. they had a r160 with the 3.9 ur looking for in the open variety stock from the factory. http://www.northursalia.com/techdocs/trannychart/trannychart.pdf
another good link of some diff years and what they have for diffs. http://kmhafer.datsun510.com/Subaru.htm
Still need a diff and driveshaft? I'll have a VLSD 3.90 R160 and GD wrx automatic driveshaft available after this weekend.