I have a 1968 cougar 289 2barrel. My Grandpa rebiult it and found out that the factory put on high performance heads on the engine. Would this make my car rare at all or raise the value?
Not unless there is proof that the car was built that way which will be near impossible to produce.
Most likely you have a 302 block and heads with a 289 crank and rods. This was common towards the end of the 289 production run.
What makes you believe the heads are “High Performance”?