You're right--265/35R18 would be a better fit than 265/40R18. That was a mistake on my part. Anyway, I have time to figure out my tire decision; I'm not really worried about that at this point.
What I could use help with is my question about the wheel offsets. I want to get the ARC-08 wheels in 18x9.5". The two offsets available are ET35 and ET22.
ET35: This what I'm leaning toward. With the help of spacers (5mm front and 12mm rear), I can get the wheels almost back to the exact OE centerline - within 1mm in front and 2mm rear. The only downside is that it requires spacers to make this correction, but I really like that I can pretty much get the wheels right back to where there are with the stock setup. I don't want to change the geometry any more than I already am by going to an 18" square setup.
ET22: The downside with this setup is that it pushes the wheel centerline out from the OE geometry, adding 0.7 inches of width in the front (9mm on each side). I know a lot of people like the fact that this wheel widens the front track, but it's still a change compared to the OE geometry.
Both setups have their pros and cons, I guess. It's just a matter of which square setup is best overall for what I'm trying to do.