The process that made the "splotchy" dark and light areas, called spangles, still exists big time. The spangles were produced on a high volume, continuous hot dip galvanizing line at a coated sheet mill. The sheet was then shipped to a stamping shop that made the part.

Back in the muscle car days, it was normal to make the galvanized steel sheet with very large spangles, because most customers preferred an appearance that was easily recognized as galvanized sheet rather than uncoated steel sheet.

Nowadays large spangles are not produced as much, and galvanized sheet is usually smaller 'regular spangle', 'min spangle', or 'zero spangle'.

Sometimes you can see large spangles on such things as traffic signal poles that are hot dip galvanized in a batch process rather than on a high speed line.

It's pretty exciting to stand next to the molten zinc pot with steel sheet running in and out at about 100 mph. Here's a 2 minute video.

https://www.youtube.com/watch?v=X9nAtO7Oof8