In order to answer the question better I need to know the tire size, and the distance from the rear drum to the frame.
285/40/17 is probably your choice right?
That is roughly 11.2 inches of section width, and is 25.9 inches tall. Which the stock tire at 215/65/15 is 26.0 tall. So your speedo and ride height will go unchanged. But the thinner sidewall will give less traction on launches but offer better handling. So its a trade off.
You will want to roll the lip if you plan on lowering the car alot. Or get air shocks, I did both just in case the GN needed to get my gear to a gig.
I have 295/50/15's on a 10 inch wide rim on my GN and they fit fine when I rolled the lip. But I also have the aluminum drums, so that adds to the backspace calculations differently.
You have the stock drums right?
What I did was take the tire off and measure from the front of the drum where the rim is going to mount up against and measure to the frame where the tire sidewall would rub if it did. I believe I got something like 5 7/8, or 5.87. But again, that was 2 years ago and with the aluminums on.
If you get the distance from the drum to the frame, the rest falls into place.
Example, if you have stock drums and SAY FOR EXAMPLE from the drum to the frame is 5 5/8 or 5.62. Then with the 285/40/17 you would want a rear rim spacing of 4 inches of backspacing and an offset of 5.5 on the 9.5 inch wide rim. The 285/40 sidewall bulges .86 of an inch off each side of the rim. So on a 4 inch backspace, the overall amount of tire going towards the frame from the drum or rim mounting point is 4.86. The frame was 5.62 from that point, so that leaves you with .76 or 3/4 of space from the sidewall to the frame.
That is .19 of an inch more space than I have from my tire sidewall to the frame. Mine is 5.3 inches of tire towards the frame and the frame is about 5.87 from the drum. So I have .57 of an inch of space from my tire sidewall to the frame and have had very minimal rubbing. I also have new poly bushings in the arms and the arms are boxed. But still have the stock rear sway bar.
To make things hairier, lower ratio tire sidewalls like 40's and 45's actually offer more useable tread due to the smaller sidewall height. And even certain companies have more useable tread on the same tire sizes. Ahhhhh! The higher the sidewall ratio, the less useable road touching tread. But then even some companies can tweak that, so the numbers are not really solid. So I always error on the backspace side, you can always use a small spacer to get more offset, but try getting more backspacing on a minor miscalculation.
I would go with the 17/9.5's. They are going to look cooler with more of the rim showing on the offset. Especially if you go with polished billet or chrome. I love the polished billet autodrags.
Sorry for the long reply and way to many numbers.
Email me if you need more, I will try to make more sense.