When it comes time to buy a windshield, go through one of our vendors, as you're much less likely to wind-up with a crummy jobber windshield. In fact, this is one of those times when you should spend a bit more and get one made by the OEM manufacturer, or equivalent. Jobber windshields typically have a problem with visible distortion in the sides; particularly in the lower corners, where the curvature is the greatest. Our vendors know who's windshields are good and who's are bad.
Some people don't even notice the distortion, until it's pointed out to them. Then, it becomes such a distraction that they end up buying a new windshield. The OEM manufacturer is Sicursiv. I believe Sicurvetro and Siglia (spelling?) are made in the same factory as Sicursiv and are identical. OEM windshields are also available with a dark tinted band across the top, although they're a bit more expensive than the more common green tinted windshield.
Years ago, one of our fellow Pacific Northwest Pantera owners found an unusual Pantera windshield by accident. He had a number of Panteras and was one of those guys who liked to have spare parts on hand. He bought a couple of windshields from a Ford Dealer that had some NOS Pantera parts and wanted to get rid of them. Before he paid for them, he asked the parts guy to open the crates so he could inspect the windshields. He was quite surprised when he found that one of them was bronze mirror tinted. He was positive it was OEM because it was in a Ford Crate and had the usual Sicursiv logo on it. He didn't say a word about it to the parts guy, other than it looked OK and he'd take it!