Have you ever seen a stock MKIV wheel on a MKIII? This is what I use to base all of my measurements and fitment characteristics.Poodles said:wasn't sure how sunken in they'd sit...
The MKIV rear is 17x9.5 +50, and sits pretty decently flush on the car.
Picture: 285/40/17 on MKIV wheels http://members.shaw.ca/annoyingrob/Supra Pics/New wheels.JPG
a +42 will make it stick out 8mm more, which will be neglegable in difference. Spacers would not be needed, or recommended (it would make them stick out MORE)
As for clearance, the MKIV wheel sits pretty close to the stock suspension. It's more or less as close as you would want to get. So, that's about 170mm from the centerline to the back of the rim. You can use this to calculate if your new wheels are going to hit your suspension. 180mm would be about the absolute limit. It's going to come down to tire choice at this point. A tire that sticks over the wheel more is going to have fitment issues.
flubyux2 says he runs 8x10 +44 on the rear, and this is just approaching the limits of rubbing on stock fenders. This would be sticking out 12mm more than the MKIV wheel, which is slightly more than the +8 that the 18x9.5 +42 you were talking about.