Now I understand why you want such large tyres, for the gearing. Looks like it's incredibely undergeared.
You say your diesel (I did a google search on it as I didn't know this engine code

) revs to 4400. And even with the 1100's it will only do 137 at max revs.. That's going to hurt your eardrums and fuel economy. It's not much fun cruising on the highway at 90kph with a diesel engine revving it's nuts off..
I used to have 4.7 diffs in my Landy with the Toyo engine and gearbox and it did 2900 revs or something when doing 90. It drove like a tractor. I hardly had to use my brakes, engine compression coupled to a very low gearing did all the braking for me. It was insanely strong with that low gearing but tiring to drive. I changed to 3.54 Range Rover diffs and this is a lot better. I can cruise at 100kph easily (it now does about 2200revs when doing 90), and have enough oomph left to overtake.
But it struggles a bit with hills so my plan is to go to 4.11 diffs (like in the Cruiser) and that should be perfect. That will also fix my speedo problem, I can use it all from a Cruiser.
Anyway,
If I'm correct there are 2 different axle ratios with the C3 range vehicles, a 5something to 1 and a 7something to 1. IIRC the former went into the C303 and the other one into the 6x6's. I don't know where the difference is, whether it's in the portals or in the diff, but if it's in the diff, is it possible to swap them? To get the taller ratio in your axles? Or do you already have the taller ratio?
What transfercase does it have? Marks4wd in Australia (
www.marks4wd.com.au) has lots of aftermarket gearing options for the Hilux gearboxes, maybe they have something to raise your gearing. Ususally they lower it for rock crawling and all. I know they do an overdrive for the LC transferboxes.
That would be an option to get your revs down at cruising speed. The price of such a thing might even be covered by the price difference between 1100's and 900's, who knows..
I don't think it's really a bad thing to have tyres that are rated for a much heavier vehicle, besides that they might not bulge out much and they could feel a little stiff and bouncy on the road. But lets face it, you've got a 6x6 that doesn't weigh all that much and if it's sitting on 1100's, I don't think you need to worry about not having floatation on the soft sand.. You might not even have to lower tyre pressures
Depending on price it might be an option to look for another brand of tyre. I don't have any experience with ProComp but I've never heard anything bad about them.
Don't forget that your vehicle weight is not that much and it's carried by 6 wheels with probably very large tyres. That means the don't carry much weight. So even if you would fit a slightly 'lesser' tyre, you might not even experience any problems.
But something like tread life is something to keep in mind too. I've got some really cheap tyres on my Landy, but now after about 12000km they are already quite worn (tread depth is less than half of when new) so cheap isn't always good. 750 XZL's will last a lot longer than this, so even if they are more expensive at first, they will be cheaper in the long run.
Can I ask why you choose for this particular engine/gearbox?
Funny by the way that your vehicle is 24V and the engine 12V, in my situation it's exactly the opposite. Landy is 12V but the 3B toyo engine is 24V, bit of a challenge to get it all to work properly. And having dual batteries means having 4(!!) batteries..
If you had chosen for the 13BT engine that would have been a bit easier I reckon, it's 24V for starters, and as it is a Cruiser engine, there's is LOTS of aftermarket gearing options. You would need the 5speed with split transfer case, but if you find a 13BT complete with gearbox and all it already has that.
Then you can get other transfercase gears, overdrives, underdrives, dual transferboxes, the whole lot..
I must admit, there's lots of stuff for the Hilux gear- and transferboxes as well
