Hi,
Congrats - beautiful rig. A long time dream of mine.
About the suspensions - I have a few remarks -
First, about the rig's height - since you guys don't have a lot of boulders in your area, but you drive a lot in sand, meaning you develop a high speed, try to keep the suspension not as high as you would put on a rig meant for boulders. However, remember that in sand the up travel of the suspension is more important than the down travel so the suspension has to have some height - just not too much.
As for the rear suspension - if I were in your place I would check if you could find leaf springs for your target, in Australia. You should make sure that it is an existing product with a lot of experiences behind it, since you will be without local service for it. It's important to estimate the car's weight correctly without exaggerating.
As for the front suspension -
As far as I can see from the pictures and the video (cool video, btw), you have to fix the castor alliance after a more than 3" lift, and I highly recommend you change the panhard to an adjustable panhard.
Judging by your tyre size, I don't think that the 2" lift from OME will help and be as effective as you expect.
Apart from that, in high speed sand driving and driving in general - where you want to keep the momentum as high as possible and not lose your grip, the most important factor \is the shock absorbers (mainly their up travel). The EFS shocks are very low standard. I highly recommend you switch to FOX or KING shocks (like you have in your FJ Cruiser).
Fox don't have shocks specifically made for the 76, especially not with such a lift, you will need the shocks to be custom made but it's not too expensive and doesn't take long, I work with this a lot in Israel - it takes me a day or two to prepare shocks - adjusting their ends to fit the vehicle mounts and the valving to fit the vehicle's weight and purpose.
I see that you travel in Jordan which is close by to here. I don't know if you can, but I would be more than happy to invite you to Israel for a few days, I could help you with the suspensions, and take you on a trip to see a bit of the country.
Cheers,
Erez