It's kind of an open ended question, and a lot would depend on your goals for the car.
If it is fairly original now, decide how much that matters to you, and if it does, then consider trying to repair/restore/preserve parts on the car, which you can do for many (not all). You'll want to decide if you care about parts made overseas - if that's fine, then you should be able to buy nearly every single part from parts catalogs like NPD. If not, you'll want to find other sources, maybe mustang specific swap meets, ebay, etc, to find NOS or good repairable original parts.
I'm starting from a gutted shell with a pile of highly worn parts, so I'm trying to preserve the ones I can, but am freely throwing away a lot of others (mostly suspension bits which I'm replacing, for example).
I am patiently looking for a 1965 or 1966 fastback to be used for a coupe conversion, ultimately becoming a vintage racer clone for myself. No need for title. I have a mostly complete 1968 convertible that I would be willing to trade. This is a parts car, no title, with many...