The following only addresses the 'width' problem -- the diameter is another matter that is more obvious, and I won't cover that herein.

Your concern is similar to one that I had regarding putting large alloys on my 240. I'll use this as an example, and then address your numbers.

My 240 has, as standard, a 14x5.5", ET=+20mm (that is, a 20mm offset), steel wheel. I put a 16x6.5", ET=+25mm, alloy wheel on it, which was rather a close fit, enough that there was almost no room between it and suspension pieces for the inside edge wheel balance weights; they had to be replaced with "tape" weights. Why?

Ordinarily, when you go to a wider wheel of the same offset, half the increased width is applied to each side (inside and outside edges), so going to, e.g., a 1" wider wheel adds just 0.5" to each side. However, I exacerbated the situation by using a wheel with a greater offset (as you have); this extra 5mm (or ~.2") shifted the centerline that much, and brought the wheel a total of about .7" inward (and only .3" outward), and so most of the increase in wheel width impacted on the inner side of the wheel, bringing it close to the suspension pieces. Combined with a greater tire width, this can be troublesome.

Now, in your case, you're going from a 15x5.5"x40mm wheel to a 16x8"x43mm wheel. Here's what's happening for you.
Based just on wheel width change (assuming equal ETs), the 5.5 to 8", or 2.5" increased, difference would add 1.25" on both inside and outside edges of the wheel. But you, like me, are using a wheel that has additional offset, in your case only 3mm more. That's not much, only ~0.15 inch or so. That means that the shift in the wheel's centerline position inward is that much, and the result is that the new wheel will extend outward about 1.1" more than the old wheel, but also extend inward about 1.4".

Ideally, on RWD Volvos (which have a positive offset), you would want (not that you can find) a replacement wider wheel that has a decreased offset equal to half of the increase in wheel width, in order to keep the inner edge of the wheel the same distance from the suspension.
[This ignores the other side of the coin, that being clearance with the outer fender, something 'Walrus' mentioned; as well as the additional concern from reducing the offset that might overload outer wheel bearings, thus suggesting keeping the offset the same].

Take note that on my 240 (admittedly a different suspension), a mere 0.7" inward intrusion reached a practical limit (any farther, and the wheel would scrape on the suspension). You are contemplating bringing the inner edge of the wheel almost one and a half inches inward -- that could be a serious problem, depending on your 164's suspension.

Although I have a 164 (a '75), I haven't tried to fit my 240's wheels on it, and I don't know if they would have a problem. Also, my '75 164's suspension is not quite identical to previous 164's such as yours -- for one thing, my '75 has a standard 20mm ET (like later 240's, but with a different suspension), but all earlier 164's has a 40mm ET as you have noted.

Be cautious.

Good luck.