All Aussie 60 Series Cruisers came with 4.11 diff gears. If fitting larger tyres and you want to keep the revs the same you can replace the crown wheel and pinion in each diff. Below are charts to calculate engine revs. Work out what revs you are doing with your current tyre size and then use the chart to find which gear ratio you need to keep the same revs with larger (or smaller) tyres.
For example, if you have 31" Tyres and a 5 speed manual, your engine revs will be approximately 2744 rpm at 100km/h in 4th gear. If you step up to 35" tyres your revs will drop to approximately 2431 rpm. To get close to the revs you had before, follow the horizontal along the 35" line where 2697 rpm is the closest. Following the vertical line you will find that 4.56 diff gears will give you the closest revs to what you originally had.
You may find it easier to simply use a maths equation; 35/31= 1.13, 1.13 x 4.11 = 4.52. So a diff ratio of 4.52 will give you the same revs. 4.52 isn't an available ratio, but 4.56 is, so it will be close.
The revs in green are the various stock tyres that the 60 Series came with; 205R16 = 28"; 7.50R16 = 32"; 31x10.5x15 = 31"!
Another alternative is to fit new transfer case gears from Marks 4wd. They have several different sets of gears available with differing ratios. These gears will change your high and low range gearing, therefore you can have low range 'crawler gears' on your 60. Be aware though that many people complain about the noise of these gears. On the Marks website they also give warning about how loud the gears are.