2) can I just build up to the RSJ (if so how would you attach the
ceiling plate to the RSJ)?

Is there any need to attach to the ceiling? I'd just mount the studs firmly
against the walls each side and mount the top plate on top of them. I'm
presuming the "span" is quite short. Make sure you tape the plasterboard,
though, to avoid cracks due to movement at the top of the wall. I'd be
tempted to use a bit of no more nails to stick it up to reduce movement, but
not to provide actual strength.

Christian.