Unless the Ep3 seats fit the existing rails, it would need a specialist firm to do the alteration as you are interfering with the 'type' classification and your Insurers would be less than happy. If an alteration is needed to the rails speak to the Insurer and check. Even if the seats do fit the rails still inform your Insurers as it is considered an alteration. Remember worst case scenario if you alter the seat rails and there is an accident, if your passenger is injured and your Insurers were not aware you will be paying for the rest of your life.
I did an alteration to the passenger seat on our old Rover. I put rise and fall from a donor drivers seat as my Wife needed extra height to get in and out.
I was in contact with my Insurers and they were just little wary until I could assure them it was a case of transferring the mechanism. Rover had built most of the fittings in both driver and passenger seats.
