You could go one of two ways...modify the panel in the bezel to match the empeg, or modify the empeg fascia to match the panel. The problem is that the black in the radio bezel matches the rest of the Mustang trim. So, most likely I'd want to modify the empeg fascia. I could either go the paint way (like I've seen a few people do), or do the full custom fascia. I wonder if anybody would be interested in making one for me...The color and texture is referred to as "black camera case".
If anybody is interested, here's a link to a few pics of my car. I've since buffed out the oxidation, so the color looks pretty good again. Still tons of work to do on it, though. First thing to do is get a new engine, followed by a new top.
http://www.pbase.com/svferris/my_mustang_project_car