Hi there and Welcome to the forum Orange Roughy.
As previously mentioned there is a couple of links to posts regarding the Marks 5 Speeds.
In Brief they were built here in Australia, they either Used Nissan Caball or Cabstar Internals, cannot recall with ones exactly. They Stopped Production many years ago, and Finding the Caball/Cabstar internals is now going to be a bit hard.
Marks 5 Speeds do Pop up occasionally, if you go that route you will have to modify the Tail shafts take that into note, To this date I Don't know of any Marks 5 Speeds in NZ, I don't even know if they were Distributed there either.
Personally myself and other mates who have/have owned them find them very over-rated, yes the Super Low 1st gear is good in the bush, Useless on the Open Road, and often 5th Gear is to tall for an old G60 To pull unless its long flat driving.
There a rather weak box from mates of mines actual experiences, one mate blew 5th Gear Twice, Another Mate Snapped the Output shaft 3 Times, and recently another mate Snapped the input Shaft Leaving the local service Station, all 3 G60s had the Stock Original (P) 4 Litre, 6Cyl Engine.
If you want a No nonsense gearbox that's bulletproof you honestly cannot look past a good old Original 3 Speed, Especially a Well Service/maintained one, what ever route you take mate Best Luck With it
I see your a Kiwi mate, I have a Few Good mates who live over there, most of them in the South Island, met them all through the G60 Scene, Such a beautiful country you live in mate love it there, here's a post from when I was in NZ
viewtopic.php?f=9&t=4286&p=39116&hilit=zealand#p39116
I even put out some feelers and chased some leads for them as well, hard to believe I've Lined them up with several private G60 Purchases from across the ditch, all my mates call me A Blood hound
If you need any bits, Send me a PM mate and ill point you in the Right Direction for spares in NZ got a lot of contacts over there, you mention Raglan, wasn't the one the Te Mata Quarries Had ? that's one lead I didn't chase up, cheers mate
