I like your thinking. Conversions of this nature are pretty expensive. You'll typically be replacing the entire underneath of your car (fuel tank, rear suspension hubs and axle and spare wheel container etc...) and you will need custom prop shafts as the cars are (AFAIK) different lengths. Then remapping and fitting electronic diffs from the Evo are pretty hard to do.
To be brutlaly honest if I were you I'd stick with some clever diff on the FWD setup and keep power figures down and focus on mid range torque gains.