Hi Evan - this caused me some headaches too. The scrub radius is the distance between the point on the ground where the steering axis intersects and the centreline of the tyre. Most cars have a small positive scrub radius (tyre centreline outboard of steering axis intersection point), which is supposed to provide a bit of steering "feel". Some people advocate zero scrub radius, but not many cars use this arrangement.
Alfasuds use Mcpherson struts, so the wheel offset necessary to clear the strut creates a positive scrub radius. As you widen the wheels, the offset required to clear the strut increases, and the scrub radius goes with it. This can result in more tyre wear, but only in low-speed corners - when the wheels are pointing straight ahead or in fast corners there is virtually no effect.
The biggest issue with scrub radius as far as I am concerned is the increase in steering kick-back through the wheel due to uneven roads. As the scrub radius goes up, this kick-back can be quite bad. However, because the Sud also uses an aggressive front camber (-2°), the tyre contact patch will move inboard, helping to mitigate the increase in scrub radius due to wheel offset.
I did a lot of investigation into this because my Green Machine has a very poor scrub radius of more than 4" designed into the front uprights. I was thinking about fabricating my own uprights to get over this, but in the end reason prevailed and I altered the front camber instead. I still need the steering damper that I added to my steering rack, but the kick-back is much improved now, and certainly livable-with on our typical roads. I have come to the conclusion that anything less than 3" scrub radius is OK for road use, and if you are circuit racing it shouldn't be an issue at all