First off you need to prep the engine. Seeing as how all 3S-GTE's are in FWD layout, you would have to make a custom intake and exhaust manifold for RWD layout. Then theres the issue with how the engine sits. If you notice, 3S-GTE's sit at an angle in the engine bay compared to other engines. If you don't place the engine at an angle, you're going to have oil issues since Toyota designed the engine to sit at an angle. Then theres the wiring. First 3 ECU plugs you won't have to worry about since they deal with the engine. But the fourth ECU plug you're going to have to mess with. Also the COR (Circuit Opening Relay) and the fuse box need to be dealt with. Almost forgot, you might have to extend the whole engine wiring harness or cut a hole through the firewall to pass the harness through it. For the drivetrain, I've heard of people using W58 transmissions with a custom bellhousing. You might need to get a custom driveshaft and a different rear-end. Celica Supra rear-end's have been known to swap over.
I know I must be missing a whole lot of details and I could be wrong with some of the details, but I know for a fact that this engine swap ain't easy. You would definitely need time and money to do this.