IDK about Darts but a 67 Barracuda has one restraining strap running left to right. Somewhere along the way they changed to 2 straps running fore and aft. I think 68. Aftermarket tanks accommodate either year but a factory tank probably does not. The difference being recesses in the tanks stamping for the strap.
You are correct. Both the Barracuda and Dart had the single strap in ‘67. They were changed to the dual strap in ‘68. Good catch, I missed it. The OEM ‘71 tank won't fit in the ‘67.
