I haven't actually done one of these swaps, but I read up on it years ago. You probably can run your ecu on the opposite side of the car if need be. What year is your car. S4 and S5 FC chassis use diffent computers and sensors and I think it needs the cluster and underdash rewiring. I may be totally off here, but it's easiest to get the same series as your car is when doing the swap.

Check RX7club.com FAQ in the 2nd gen section