Extending track width with extra offset question

[DJR 7]

Hi guys I'd like some help!
I'm embarking on a project and I need to widen the track with of a Tuscan by an inch (25mm) both sides.
I'm not sure if this can be done be changing the wheels alone?

Currently the Tuscan runs 8.5/18 with 42 ET front and 8.5/18 33ET rear
My thoughts are: if I go to a 8.75/18 with a 20ET front and rear i will widen the front by 28mm and the rear by 19mm

My questions are as follows

A, are my calculations correct
B, will the 20ET offset work if I have a geo set up

Any input would be greatly appreciated, I'd also be interested in the offsets and wheel widths being run by others.

D