This is true, I was using the outside of the wheel cover on the Chevelle and it is most likely at least 15".
But still well within the boundaries of "close enough" for this purpose I am hoping.
By the way Fozzie I am doing the exact same thing on my truck in that I am looking at making my wheel openings the same as a 53-55 or 56-57 Vette. I am not at that point where I am doing any cutting so right now while dreaming of the project I am keeping my eye out for these cars and just standing back and eye balling them thinking about it. At a recent visit to a local museum I took these photos for the project.
I hope to literally be able to take a pattern off a real car just for kicks but if I can't find someone willing to let me I will be using photos like these.
Back before photo shop I use to blow up a photo and then cut measuring carefully a known, and chop the top or section the body of the car or truck in the photo. Standing back and looking at mocked up modifications is one of the most important things you can do when customizing a car. All too often this is done in hast and the cutting and welding is done before a real good look has been done. A photo cut up on your dining table and taped or glued together is one way to mock up your cars modification and "stand back" and get a good look at it. Now this is just one dimensional and you can't walk around and get different angles of it, but it is better than nothing.
I have one of these photos on the wall of my garage that I did of my truck 15 years ago, hopefully I will be back on the truck in not too long. Right now it's just inspiration to finish the project I am on.