120y wagons are awesome, I would buy a 5 speed if you are thinking of keeping her as she will like cruising better with an overdrive. She will always keep her value been what it is and not a Hyundai excel so its up to you.
Diff wise the 3.7 H150 is quite rare in Oz and the 3.5 and 3.3 non existent and command around 300 USD from the USA. Then you have to have it fitted and will cost halve of a 5 speed and then you can just use the 4 speed 60 series box but they are rare too in good condition.
Someone I think Bige was selling a datsun 60 series 5 speed with all the goodies and I think Nick might have sold his Toyota t50 dellows conversion also or not selling anymore?.