16" wheels for a 67 barracuda?

No they will not work. You say the bolt pattern is 100mm. At 25.4mm per inch, 100mm/25.4 mm per inch = 3.937 inches. They will most likely mount over the wheel studs OK, and you will probably even be able to tighten the lugs. However, since the bolt pattern is 3.937 inches and not 4.0 inches, the wheels will not be safe. They would be off by just a teeny little bit. Trust me. Even if they appear to fit, they will not fit properly.

Agreed. 4" =101.6 mm. And like others have said I wouldn't run anything but the exact size.