I'll offer my two cents, on two topics. One, if you are going to run the car on the street and truly want to enjoy the ride, I'd go with as large a sidewall as possible while still hitting the look you are after width-wise. Two, the most importnat thing you can do to make your car truly awesome in the rear is to install a Lateral D three link setup!
Okay, so I'm biased... Honestly though, tires are SO good these days, you probably wouldn't notice the difference in performance between an 18, 17, and maybe other rim sizes, stressing performance. In terms of ride quality, the more sidewall you have, in general (depends a ton on tire construction), the better of you'll be.
One truly hard thing to do is to find a really good, huge tire for the bigger cars, like the 68 GTX we're doing, like yours, there are few choices for tires on this one. Aaack.
Mark