'67 Fastbacks are my favorite all-time Mustang!
Having said that...... no way. For $3800; you're buying the VIN tag (if it's still there), the back glass, and maybe the tailight assemblies. Pretty much everything else is gonna be replaced. The guy should be ashamed of letting it go that far; and even more ashamed of trying to take advantage of somebody with that sale.
For what you're going to pay in parts and labor on making that rusty shell into a viable chunk of metal to make into a car; you could be an entire re-pop body, already assembled and no rust. Lots of controversy in that idea; people claiming it wouldn't be a "real" Mustang - but, with all the new (re-pop) sheetmetal that you'd need on the shell in the picture, very little of the "real Mustang" would be left when you're done.
Something to think about.