That's not really accurate information. Both of my '71 Dart's are 318 cars, and they both had 1/2" eye bolts. I don't know when, or if, the V8 cars started using the 5/8" diameter eye bolts, but I'm pretty sure you can't count on it for '71 seeing as how my cars were built in different plants at different times.
Most of the aftermarket springs use a 5/8" eye bolt, there are a couple of exceptions.
