Since you're a few thousand miles away, it's kind of hard to give good advice. LOL. I don't know what type of GM trucks were available in Australia, but from a little research, I found that Holden did use Muncie 4 speeds, like their American counterparts. The Holden Commodore V8 did also use a T5 transmission at one point too, which had the Muncie style bolt pattern. It looks like the V6 commodore used the Ford style bolt pattern. The early Toyota FJ used the Muncie style transmission too, but had different front bearing sizes and throw out bearing sizes were different, so it might be a pain getting them to work correctly.
For an open driveline rearend, I would look for a Toyota rear diff. Here in the states, the 95-05 Tacoma is about the perfect width and the 4wd axles were 6 bolt wheel pattern.
Look around for some Hot Rod shops or 4wd clubs, they can probably point you in the right direction.
