There are variables that go into the appearance of orange peel. Remember that these cars were painted with acrylic enamel and then baked to cure, so the paint would flow out and have a more 'coarse' appearance, the orange peel looks 'bigger', for a better term. Most painting done in professional shops these days would not be baked like done at the factory. This ends up requiring the perfect combination of paint mixture, spray method, exterior temperature, humidity... to replicate the original orange peel appearance. I'm by no means a professional painter, but have discussed this at length in the past with my paint guy, who has been running production body shops for 35+ years.