In addition and as posted in the large thread covering the color the Dark Charcoal metallic is often a thin paint without allot of pigment so many (including me) apply it lightly (a couple of light dry coats) over a base of black. With many of the mixes and without a dark base by the time you build up the color the paint layers can be thick, filling the grain, and can produce a glossier than called for final finish
Back to your question - yes it would help knowing the color of your interior or the code. Of course if you look in the Library section of the site under 1970 Mustangs you will find an article that includes the interior paint codes, and the Ditzler code fro that color (from the 1970's) as a start. Paint can be difficult since not all of us can purchase the same products because of state or country legal issues and painters that will not choose to run the risk of spraying illegal paints for good reason