All the information you will find will be anecdotal based on people's experiences and opinions. Back in the day, most people paid no attention to these things as the cars looked "ok". Panel fitment and panel gaps are a relatively recent issue or "thing" most likely as a result of all of the car shows, YouTube, etc. Not saying it is a bad thing, but just trying to give some prospective.
Original fitment and gaps varied greatly car to car overall; given that they were brand new when built there were guidelines during the assembly process and things had to fit relatively well because they had to sell the car and if it looked like crap, no one would want it. Point being that the people doing this for 8 hours every day for years I am sure had a "feel" for it and was able to slam them together very quickly. I am sure that there were some QA standards that were checked but having owned a number of them and having been around them for a very long time, all things are relevant meaning that most people don't pay any attention unless it is horrible.
All that said, 40+ years later, having been rode hard and put up wet, things tend not to want to line up as they did when new which requires some massaging to get them in the ball park. Many people want the panels to be gaped equally everywhere which takes some effort but it does look nice.
When I built my 70 Challenger (2000 to 2004) I built it as I did back in the late 70's/early 80's and to be honest paid no attention to the gaps and thought they were good. When I built my Cuda 2 years ago I paid a lot of attention to the fitment and gaps and at one point went to the Challenger as reference and realized that the gaps on the Challenger are horrible in comparison to cars built today (factory or custom), point being for many years I never gave 2 seconds of thought about the panel gaps until I did... If I ever redo the car I am sure I will pay more attention to them but to be honest I don't think it makes a lot of difference depending on the type or level of build.