I would think it would be hard to find an extremely clean original 6 Fastback. That does not mean that they are valuable. Many of these cars have been converted to V8's for the obvious reason that the market is bigger, and they are easier to sell.
I have a 68 FB that was originally a six. Great car, tight, unmolested. A six could be interesting though, particularly if you went to a company like Clifford to see what you could do to increase performance. The assumption here would have to be that you got it for a VERY good price.
Wow, amazing! I guess the real intrigue for me is that it looks like a performance car but has the base model engine... Kinda neat.. I would wonder though after always driving 289's, would i be dissapointed in a 200 "6"
You may want to check this web site http://www.fordsix.com/index.php, which is dedicated to performance I6 cars, a lot of which are Mustangs. I have an I6 in my 66 mustang converitble that was rebuilt using a "warm" cam among other performance goodies. The increased performance is pretty stout and lightening fast on the road. It has plenty of pedal regardless of how steep the grade, and the result is an enjoyable ride with plenty of reserve pep at impressive mpg. The fun of driving this car is not diminished an iota when compared to V-8 cousins, but most if not all of them would have a hard time believing it. For me it is arguable that within the law and bounds of safe driving my I6 delivers better performance and just as much thrill as any V-8 - but then that's just me. I also own a '56 t-bird with a stock 312 that tears up the road, and so I do have something to compare with.
Had a buddy in college back in 1970 with a 1965 coupe with a 170-1V inline six and auto. It was sooooooooooooooo slow it was a joke. He once got it up to 75 mph with 4 of us in on a 10-mile stretch of I-95. That was the only car he ever owned that he never got a speeding ticket in.
I bought mine a few years ago 200/4spd combo for 4500, in basically the same shape as the sig except the bumpers, misc trim, and the obvious v8 swap. The 200 was a turd, my friends re-named it the generator.