Yep 4 link would have been my suggestion. That FD rear end is a full blown double wishbone, that would be an epic feat.

Subaru brumby's and 4x4 wagons are IRS. They aren't a very wide car, either. I'm sure there diff's aren't the greatest, though. Could possibly get hold of an RX turbo diff.

Lots of Subaru stuff runs the r series diffs like the 1600.

5 link - including panhard bar/watts link. For what they are their very adjustable, the only things that can't really be done is camber and toe adjustments, unless the diff tubes are hit with an oxy torch.

 

4 link - is where the top two bars are mounted to the diff near the centre and come out on an angle to the outside of the car. Adjustable to a point just not as adjustable as the 5 link. The only thing going for it is being compact and less fiddly in setting up.

 

(Pictures are from car bibles)

i know you said your heading away from IRS, but what about the NA-6, NA-8 or NB MX5 rear end, from what i know the NA-8 came with an LSD stock, just an idea i thought i'd throw out there...