As I'm kinda sad like that, I have a small spreadsheet to calculate such things as offset, tyre size etc...
Going from the standard 7" wheels with and offset of ET52.5 to 8" wide with an offset of 40:
Inner edge is 0.2mm further in - not worth losing any sleep over.
Outer edge is 25.2mm further out
For comparison, my alloys sit 15.5mm further out than standard, plus I have 12.5mm rear axle spacers, so 28mm in total at the rear:
The only thing to bear in mind is that wider wheels will tramline more, and I've found the wider track makes the steering a bit heavier.
You will need to lower it too if you haven't already, probably on coilovers so it's more than I have (H&R springs only) or it'll look like a monster truck!
Will look the nuts though once it's all together!