It depends on what you mean by "will
it fit". I'm pretty sure the GQ and GU LSD's are the same so my experience should count here.
Firstly, I'm not sure that the H260 ever came as an open centre, yours could just be a very worn out LSD, but
@james008 is the expert there. If you open it up and it is an open centre I'll bet that'll be something no-one's ever seen before lol. Maybe it's a version that was never available in Australia.
If you want to fit an entire GQ centre into an MQ, have a look at this thread:
I swear every time I take out a diff something goes wrong. Anyway, as I'm currently putting a GU H260 carrier into my MK housing to get a 4.375 ratio, I've come across a problem that could end up being quite big, or could be an easy fix (probably not). In the MK diff, the way the wheel bearings...
www.patrol4x4.com
That'll show you how to fit the carrier into the MQ housing.
If you're instead looking to replace your open centre with a GQ LSD using the same ring gear bolted onto the new LSD, you'll have to drill and tap the ring gear as the later 260's used slightly bigger bolts.
The main mission will be swapping the metal block in the middle of the centre over. You'll need it either way since the MQ rear wheel bearings don't work independently of each other, each can only support load in one direction and relies on the axles sitting against a metal spacer in the center to transfer sideways force to the other wheel bearing.
So either way, replacing the whole 3rd member, or just the lsd centre, you'll have to either be comfortable taking the LSD apart, or get someone else to do it.
Feel free to write it up if you do go ahead and do it, I'd be curious to know if there's a difference between the 2 GQ and GU diffs.