Have had to do the same on My Nav (for bigger tyres though). I am now going to try bigger tyres and if I have too cut and weld the body work.
I Can say from experience Off set is very important in fitting a larger wheel to a vehicle, I assumed greater off set would allow bigger tyres, but this is not the case as the tyre does not sit in the well at lock. I am changing to wheels that have less off set to fit bigger tyres.
As far as I have seen 16x7 steels all have the same offset as do 15x7.
HB I have a uneven worn but still legal 265/75/16 BFG At tyre if your intrested as a spare.
For the Record 16x8 modular Jap steels are 0mm offset.
15x8 8 spoke Jap steels are -25mm Offset.
15x8 modular Jap steels are +10mm Offset.
15x10 Jap steels are -32mm Offset.