The RA40 was designed with very similar mechanicals to the RA23/28, so I would *guess* that yes, the diff and springs *should* fit, however, the interior was completely redesigned so you have no chance in heck to squeeze a RA40 dash into a RA23. The centre console could be made to fit but you will have to do something *clever/custom* for the mounting screws/bolts.
Good luck with it all mate.
Cheers,
Mitch.