A body gas tank.

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.

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.
I'm not convinced it wont fit, it just wont be restrained correctly.
I modified my 67 barracuda to accept the two strap version. I used the trunk provisions from a 70 dart.
I cut the slots in the frame cross-member for the other end. I'm a little worried about the cross-member because I think the factory reinforced those areas where my 67 is not.. yet.