I've got a rebuilt Spicer 5831 3 speed brownie box with a .74 OD (IIRC) and a 2.0 under drive that's been taking up space in my shop.
It would work well for your application. Even have the drive shaft flanges for it. It shifts with a 3 speed aftermarket shifter like a Hurst!
Was going to use it in my FERD Class A Motorcoach but changed my mind and went with an Allison World 1000 5 speed behind the 5.9L Cummins I put in it.
PM me if you're interested and I'll shoot you some pictures and the spec sheet.
One of the nice parts of it is the under drive 1st gear for slow going.
Like pulling a boat out of the water. or maneuvering up a tight steep trail.
Woa, just noticed you are in Austrailia.
Shipping would be a killer.
But, I can still give you the specs and pictures if you want to consider looking for one closer. if you're interested.