I've just gone outside in 3degC and measured the track width of my singles on my Canter. The front track (centre of tyre to centre) is 1725mm and the rear track is 1755mm - this has been achieved with a +90 ET (offset).
I forgot to factor in the 10mm thickness of the mounting plate when I got my rims made, so that's 20mm of the difference I have between front and rear. The other 10mm would be taken care of by making the offset +85 (or +87.5? - I've had a big Xmas roast lunch and can't think straight!) But I've not lost any sleep over 30mm.
I've got about 1.5 inch space between the steering ball joint and my 17.5x6.75 rims (265/70-17.5 tyres) - whether a 17x9 with +90 offset woiuld clear the knuckle I don't know.
From memory, the standard 6-stud Canter offset is +127-129.
People get confused between offset and backspace. Below is offset, both positive(+) and negative(-). Regardless of the width of a rim, the centre is always at the centre.
Someone mentioned above that without the inside dual on, the bearing loading would be severely compromised. However the axle is rated to 6000kg, and as the cab-chassis rear axle weight is 1000kg and the permitted payload is 3600kg, the axle will only ever see 4600kg max - plenty of redundancy there.