I have seen 315/35's on a factory corvette rim (not ZR1)... although not ideal, because the tire is wider than the rim, it does fit, it looks a little funny though, because the sidewalls are angled in... hard to explain without a picture. I assume this would be ok for drag racing purposes. If you do more autocross style then it would be a bad idea
Speaking for the '93 model year, the rims were actually 2 different widths. 8.5 in the front and 9.5 in the back. I don't prefer this look myself (9.5s all the way around look great) but that his how they came.
As far as width on the tires, I have seen 315's on the back, but they stick out about 1/4". They look aggressive, but keep in mind that they will provide you more stone chips than normal.
Some '93s came with 9.5" all the way around. They came with the the heavy duty suspension option.
You could always add the fender flairs for the Grand Sport. That would keep the larger wheels/tires a bit tucked in. If you change to the 11" wides in the back, make sure to get the right back spacing on the wheels.