Time consuming but did this with my 67 grill. Clean with diluted Muric acid. This wil get the metal cleaned of dirt, paint etc. Dd this in a well ventilated area. Wear good rubber gloves. Water will dilute the acid so make sure the grill is well rinsed.

Go ahead and paint the entire grill with flat black. Once it's dried the hard part begins. Use a lot of clean rags. Take cue tips dipped in a paint remover and clean off the areas of the grill that are suppose to show bare metal. Very tedious but you'll like the results. You may have to touch up using a paint pen. You can get that at a hobby shop.

Some others may want to chime in with different techniques.