I might be wrong here but aren't rst clutches compatible with mtx boxes and flywheels not ib5? I vaguely went down this road when I had mine years ago is the rst clutch 220mm and ib5 210mm or something daft like that?

I can't see why you want to use an ib5 box with an rst clutch unless its turbo'd? In which case I really don't know why you're using an ib5 they're utter shit and made of chocolate