The other excellent point (missed above because I updated as Fbody posted) is that spark plug heat range is not a one size fits all.

Not only have we owners changed lots of things on our engines (a/f mixture, compression, camshaft, intakes, cylinderheads, etc.) but the fuel has changed since these cars were developed as well. As a result what worked in 196_ on these engines may not be optimal for today. Make sure your carburetor is properly tuned and then experiment with colder heat ranges. As long as your spark plugs are not fouling under light driving conditions you should be running the coldest plugs possible.