Best bet is using Granada rear seats, the cosworth items require a rear parcel shelf support that only the orion or saff has. If you use the back seats from a Granada the bracket setup is similar and easy to mod to fit. The base of the seat needs to be taken apart and put on to the base of an ERST item.

As said Jano's RWD S2 has a cossie floor pan so fits the std cosworth seats as it was designed to.

Hope that helps.