IMO, the typical Subaru engine most will run into, being either the phase 1 or phase 2 EJ22, just doesn't make power in a useful range for a truck. The phase 1 EJ22 (I think between 1990 and 1998-99) made ~135 lb-ft at a SCREAMING 4800 rpm. The phase 2 EJ22 in my 99 Impreza makes 149 lb-ft at 3800 rpm. I know I'd hate it in my BII with the M5OD.
As far as the other options go, how 'bout the EJ257 from the STi? In the stock turbocharged form it does somewhere around 290 lb-ft but I can't find the engine speed at which it develops this torque. I imagine being a turbo'd boxer 4 it is in excess of 3500 rpm.
Personally, I'd like more low-end grunt than that.
As far as the rest of the drivetrain goes - I can speak from personal experience that the CV axles, and the wheel bearings will not like the extra weight. The 4EAT auto trannies behind EJ22's probably would love to chew up the AWD clutch packs. I don't know how the viscous coupling for the 5-speed AWD system would fare.
The driveline from an STi would, in all likelihood, survive the experience. I VERY seriously doubt there's any reasonable way to get her to flex thanks to the CV axles. They're made to handle, not do any serious offroading.
If you do it though, PLEASE post lots of pics! I love anything Subaru!