Yes you can use the Legacy Auto tranny, but you'll need to also get the rear diff(to swap the ring and pinion into your LSD Diff), and the Legacy flywheel & Torque Converter, All Legacys have a lower gear ratio ranging from 3.90, 4.11s, 4.44s, the 4.11s being the most common.
I believe this will make the SVX a much quicker car, though you'll lose the capability to cruise at 155mph
but I never go that fast anyway.
It will also lower the stress on the tranny, which should make it last longer....but don't forget to install an aftermarket tranny cooler!
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Huck
Subaru Ambassador
92 SVX LS-Tour Magnaflow Exhaust, 5-Spd-AWD
88 XT6 AWD 5-Speed "Bride of FrankenWedge"
15 Impreza Premium Sedan
15 Crosstrek XV 5-Speed
My 5-Speed "How-To" Write-up
1976 Pontiac Firebird Formula
Current Count of Subaru's Owned.... "70"