Cosmetically...shoulder belts don't look good in classic mustangs. The fact that there is no B-pillar makes them look out of place. That being said, of course a shoulder belt is much safer than not having one and so cosmetics should probably take a back seat there. It sounds like a 67 is better match for you than a 68 honestly. In my 67 Coupe I actually bolted in some shoulder belts from an Isuzu impulse with the only mod needed being to cut a hole in the headliner. Of course, the downside is that those things flapped in the wind like crazy with the windows down since those particular belts didn't have the drop-loop or whatever its called....but thats just an illustration of how simple shoulder belts are to install in 67+. Also, while the 67 had an ugly steering wheel, at least it wasn't as ugly as the 2-spoke 68. In all honesty though, none of your listed reasons(shoulder belts, headrests, or steering wheel) should prevent you from buying some other year Mustang...these are small one-hour modifications you can do in your driveway after all(there a billion seats with headrests you could choose to swap in). The only year-relevant modification would be if you decide you want a big block engine, in which case you want 67-68. If you ask me...out of all the 65-70 Mustangs, 67 was the best balance of looks and desirable features. The downside is that 67-68 cars are more expensive than the 65-66 cars.