An EJ208 motor as in from a BE/BH GTB/B4? Are you intending to do a turbo conversion on the RX?
In terms of physical fitment its very easy, just bolt it in. However, you will need the wiring loom and ECU to match the motor, and you will need to merge/splice that wiring loom with the one in your car at the moment, and use the 208 ECU (assuming that you are going to use it in twin turbo configuration). If however you intend to use it in single turbo configuration, you will need a wiring loom and ECU from a single turbo car. I believe you'll also need to block some oil lines and such from the heads on the EJ208 since it's setup to run two turbos.

If you intend to use it with NO turbos, then you are wasting your time and should sell it since it produces less torque and probably less power without any turbos bolted to it than the EJ25d in your car at the moment.

As for the gearbox, you can bolt a 4 bolt gearbox to an 8 bolt block no problems, you just don't use 4 of the 8 bolt holes. It is more difficult (although still not terribly difficult) to use an 8 bolt gearbox with a 4 bolt block.

Also does it matter that the gearbox came out of a front wheel drive?