[Tireiron]

Where are you coming across a 3.70 in the US? There were almost no Subaru's built with 3.70 in this country. Almost exclusively limited to the 06/07 WRX manual.

Your best option aside from getting an overseas SVX 3.70 rear diff is to swap the r160 3.70 ring and pinion gear into your SVX r160. You cannot swap the WRX r160 in because the SVX axle shafts will not fit the WRX diff, and the WRX axle shafts will not fit the SVX rear knuckles. You maybe able to find a used diff to get the gears from, but if not you can still get them new from Subaru and I think ralli spec.