[NiftySVX]

It is pretty simple, grind off the teeth on the pinion gear for the front drive and fuse the transfer clutch together. The rear drive section of the car is dramatically undersized for a 3500 lb 230 hp vehicle and I doubt that it would hold up to 100% of the power for long, especially if you were hard on it. The rear diff isn't even mounted all that securely to the subframe. And of course the suspension is not set up for it which is why the car is tail happy when RWD.

I would suggest repair of the front diff instead of a m/t swap unless you just really want a manual. Subaru's manuals are slush boxes and the ones that will fit, with the exception of the turbo legacy, are not strong enough to hold up to the engine long term