Just read through this thread and thought i'd mention that i've seen an RB26 power Impreza before; so on this basis a 2JZ shouldnt be an issue in this car (but may require some firewall/bonnet/bumper work). Also i've seen RB26's and 2JZ's in RX7's - now thats a small engine bay!
At the end of the day, with enough money and time you can get any engine to fit any car.
I've done Toyota UZ V8 swaps before now so im looking at the room in the engine bay in the Subaru clear Prototype and i can't help but think about how straightforward it 'should' be building a V8-86
I think you can comfortably go inline 4 without upsetting the car too much, so I would really love to see a 3SGTE and a 1UZ/3UZ go into an FT86 at some point. But i think a straight 6 would be too long for this chassis and totally ruin the handling. Too much engine in front of the suspension mounts.