I too have done a comparison of an AMD quarter skin to Goodmark for the 70-71 Dart
In the photos you can see the differences.
I own both panels.
You can see the AMD panel has the dogleg below the door as original, but the Goodmark is cut straight down.
The side marker stamping is crisp where the Goodmark is crude.
The folds on the wheel opening are crisp on the AMD panel like original and very rounded on the Goodmark.
AMD panel weighs 21lbs on my scale.
Goodmark panel weighs 17 lbs on the same scale.
AMD panel is noticeable heavier, stiffer.
The Goodmark panel has huge flanges that need to be trimmed down, yet it is still 4lbs lighter.
I have installed both panels over the years on several cars.
No comparison. AMD panel is so much superior.
Perhaps Goodmark has made a new and improved version of their Dart skins since this one was made?