I don't know enough about this to say for sure either, but you will find a W50 gearbox from a RA23/28/40 will suit an 18R, and my guess is that if your bellhousing is a Y series (petrol Hiace/Hilux etc ???) with the W40 4 speed manual (as standard ?) then a W50 *should* bolt straight on.
Mitch.