Ok, armed with new information on factory diameters from:
http://www.corvettefaq.com/c3/MoreBite.pdf
I now know that at least some years, factory tire was a 225/70/15 with a 27.4" diameter all around. I would like to be closer to that diameter, but the rear tire for the 20x12 seems to have really limited choices.
Here is my other option - Michelin PS2
Front - 19x10 - 285/35/R19 26.9 diameter
Rear - 20x12 - 335/30/R20 28 diameter
I am not really sure I understand how to read sidewall height and it's not listed in the specs. Would this combination look ok in terms of sidewall height and the front not being a wider sidewall than the rear?