Matt,
As one that has resto'd many a 70-72 Duster, if I wanted max bucks out of it, I didn't even look at the 225/318's. I looked for and aquired a real no bologna "H" code "A" body. That's where the money is. Unfortunately, their are not many around and if you locate one, it's already been resto'd and garner's 15-24K depending upon options,color,etc. For your's, I would agree around 9-12K after your mod's....john