If it's a '68, then it could be in all likelyhood a 289 with 302 heads. '68 was the transition year and it could have come that way from the factory. My '68 had 289 badges, but the engine was a C8. Wrong engine, or wrong badges? Maybe neither. I've heard of C8 blocks being built with 289 rotating assemblies from the factory, at least until the parts were used up.
The other possibility is that the heads came off some other 302. If their part # is C8 anywhere, then they are either original or at least of the correct vintage.
True. They are both about on-par with the later E7 heads. However, the 289 head would have a 54cc chamber and the 302s a 58cc. Any of them can be ported to flow comparably to untouched Edelbrock Performers. 220/160cfm seems to be the norm for these heads.