Depends on what year and plant I'm trying to reproduce (there are differences
For the 68 and up Dearborn and NJ (think that is what your working on) a good high buildup black undercoating will work. May take a few passes to build up the right thickness and texture.
Pick up a few rattle cans of what is locally available and do some test on some vertically held cardboard to see the effect. The challenge is that allot of today's products dissolve the earlier coats as you add a new coat so sometimes chilling the product or letting it fully dry between coats helps
Some have had success with the Wurth or Lord Fusor products (same ones they are using for sound deadner replacement) - again test before you spray