I also like the Dark Moss Green color, it and Highland Green are my favorites on the 67-68 fastbacks. Must be that Bullitt thing. I'm not sure about the red interior matching though. I assume it is in good condition and thats why you want to stay that color. I have seen Black with red interior and it was really sharp. It is your car and if thats the combo you want, go for it.
Personally, with the red inerior, you can't go green exterior. Better to go black exterior or switch over to black interior. A buddy of mine used vinyl paint to go from tan to black and it came out very nicely. Don't think you can paint head liner though. Hmmmm...go black outside or maybe gray. Did they make a gray in '67?
according to Larry Dobbs & Larry Farr book " How to Restore Your Mustang" the available interior for '67 with dark moss green is listed with black, saddle, ivy gold, and parchment for both std bucket and interior decor group option bucket seats. it has to look good to you - personally i do not like bright garish colors but that is me. my choice would be ivy gold in your car but that is only mine.
also just because that was availble then does not mean you have to use those colors.