IMO reducing the highway revs isn't the answer, you'll run out of power to pull the higher speeds with the larger muddies. I've tried that and it wasn't great. I say put the muddies on and see where you want to move your gearing then and don't be scared of revving it.
With the stock F78-15 tyres you've got 4966rpm @ 100 with a 32.364 crawl ratio.
With stock gearing and 30s you get 4519@100 and obviously, again a 32.364 crawl
Stock gearbox+diffs, 4.9s and 30s 4659 and 62.756
As above with 6.5s 4840 and 83.752
6.5s, 3.727s and 30s 4389 75.948
4.9s stock diff gears, 5 speed (0.864 5th, there's a 0.794 5th version but it's much rarer) and 30s 4026 73.082
6.5s, stock diff gears, 5 speed and 30s 4181 97.533
Add 4ish% more revs for 235/75r15s (29.8").
The 5 speed conversion has the double advantage of not only having an over drive gear but it has a lower 1st gear too. Having said that IMO 4 speed ratios are much nicer to drive, the ratios are quite close and evenly spaced where as the 5 speed has quite a big gap between 1st and 2nd.
If you're planning to run the WT diffs you wont be able to run the NT diff gears in them.
FWIW I also have an F10a in my LJ and it cruises at 5900rpm @ 110. It's happy doing that for hours on end, wont lose speed up all but the steepest hills and will pull past where the needle stops moving on an LJ (140ish) but BYO earmuffs.
As you've got a Maruti that ran F78s (27.4") stock, 235/75r15s would actually be legal in most states depending on what other lift you have.