The difference is with the larger diameter rims. A 17" rim clears the ball joints and control arms with a stock spindle, so you can run more backspace until the tie rod ends limit you around 5.6" or 5.7" on most rims. With 18" rims not even the tie rod ends are an issue with stock spindles. But with drop spindles, that steering arm is moving 2" with respect to the center of the wheel. Which makes your rim clearance more like a 15" rim. So you don't get to run a big backspace with the larger diameter rim anymore. I was able to run a 17x9 with 5" of backspace when I was running the 2" drop spindles, but that was totally maxed out on tie rod clearance. That was on my Challenger, you'd need more backspace to fit a 9" on an A-body but the 5" backspace limit would be about the same. It's a little better than what you could do with 15's and stock spindles, but no where near what you can usually do with stock spindles and 17's or 18's.
