since I have motorsport-tech spacers i should share a few measurements i found when selecting them.
difference of (front wheel to fender) to (rear wheel to fender) is 20mm
stock gls wheels 16x6.5 +42mm offset distance to fender F 30mm R 50mm
Basically this means that you need 20mm extra in the back to make the wheels an equal distance to your fenders. Ideally a new set of rims should stick out 30mm wider than the stock 2000+ gls wheels to get them flush vertically as well. One other thing I would recommend with this setup is a wider tire up front, maybe 245 or 255 on a good set of 8" or wider rims. The alero has different fender angles front to back, so this setup makes the rear wheels appear farther out than the fronts. A nice beefy sidewall up front will fill up the difference in appearance. I'm not the best judge of performance, but I know that I have less bodyroll and more sensitive steering when I have the wider wheels on. I run stock through winter so I have plenty of time driving both setups. Also, you would need maybe 40mm-45mm spacers to clear the studs. I can't use the spacers with my stock wheels because of this. One final note is don't underestimate the importance of offset. Some wheel websites don't even list the offset, but it is the most important measurement in achieving a great looking stance. For more info on tire width/offset and a great offset calculator, check out
http://www.1010tires.com/tech.asp