There's already a 'Corvette engine into WRX' thread, but any older Impreza would be a good donor car. Lighter, better looking, and cheaper.
Why the Impreza? Why not? It is a solid little car with a few specific advantages.
1) wide frame rails
2) long enough engine bay
3) already has transmission tunnel
4) already has IRS
5) everybody (including insurance company) thinks it is a low horsepower/high traction AWD car and will freak out if you do a smoky burnout
6) gigantic aftermarket and technical support for Subie and Chevy stuff
A) So, what will be different than the LS1-into-WRX swap?
B) Is there less room between the hood and engine?
C) Is there less room between the radiator and engine?
D) Is there any difference in the front crossmember (other than the dent for the uppipe) that would cause a problem?
E) How much narrower is the transmission tunnel?
F) R160 and R180 will need to go. What should be done?
Please, please, please if you don't like the idea of a RWD Impreza don't post. We already know you don't like the idea. And you'd never buy a 5.7RS if Subaru offered one right on the showroom floor next to the 2.5RS.