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I have used 215/65/15 in front and 255/60/15 in rear. In fact have a "new" set of 215s for sale, BF Goodrich "H" rated blackwall, never mounted. email me if interested. :D
Originally, your Corvette would have been delivered with an F70-15 nylon cord tire which had a nominal diameter of 26.9 inches. The standard tire was a blackwall, but a white stripe (RPO PT7) and a white letter tire RPO PU9) were available as options. Three manufacturers (Goodyear, Firestone, and Uniroyal) provided tires as original equipment at various times during the model year.
If you are looking for a modern radial replacement for your car, 215 70R15 tends to be close to OEM size for most brands. Still, I'd check the manufacturers' specifications to be certain as actual diameters tend to vary a bit among some brands.
You might want to search the archives on the subject of "tires." Lots of opinions on this subject. I recall 225's on the later bumper years with 215's on '68 (mine.) I stayed with stock size because of rubbing, wandering and speedometer concerns. Others have opted for more tread on the ground which isn't bad, either, but comes with some tradeoffs. Good luck with your decisions :cheers: