Do Chrysler 300c rims fit a 76 Mustang?

No, Chrysler 300C rims are highly unlikely to fit a 1976 Mustang. The bolt patterns (number of lug nuts and their spacing), wheel diameter, and offset are almost certainly different. Even if the bolt pattern *were* the same (which is improbable), the offset and backspacing would likely cause clearance issues with the suspension, brakes, or body. You'd need to find rims specifically designed for a 1976 Mustang.