Would I get a (OEM Volvo) 18" x 8' ET55 with 225 x 40's on my 960 wagon?
Not easily. If you're talking those tyres of widths and that sort of ET, are you talking about C30/S40/V50 wheels or newer S60/V60?
Is yours an early or a late 960? If early, your offsets are around ET20-ET25 as per 240/740.
If late, your offsets are ET43-ET48 as per 850 and P80 platform cars.
This still doesnt address the Centrebore difference.
Of course, you can overcome all of this with a set of spacers - have the ID of the wheel centrebore on the front face and the OD of the hub on the rear face.
No real issue, but you'll need to go to some effort to do it.
Its a 96, so same as 850. There aren't many18's around for 850 offset are there? I'fe nearly got it upto roadworthy state for Vic rego. A few bushes and a front screen. Running like a sewing machine!
Thats not too bad then, a slide-on spacer to convert between the two centrebores would do it, you could get away with something around 15mm in thickness and probably.
There are a few easily available options that could be modified to suit here too.