Are you in Australia? Sounds like a weird import model. I don't think we got the TD23, Europe and Japan did though.
Looking to keep the column shift or go to a floor shift?
There are column shift diesels around but it'd be better if you could go with a floor shift and bolt the column shift linkage on, it would make no manufacturing sense for it to be different though it could be.
In floor shift, you've got the TD25 gearbox to choose from, or if you keep your bellhousing and just change the gear box you can use a four cylinder petrol box, the same five speed was present in the four cylinder petrol and all the TD motors. A 4x4 box won't be much good to you as the transfer case is married and you don't want that flange.
Also consider the Urvan, it's column shift and the diesels and Z24 petrols had the same gearbox.