* Bolt Pattern: This is crucial. It's usually expressed as something like 5x114.3 (five lug nuts, 114.3mm diameter circle). These MUST match.
* Wheel Offset: This refers to the distance between the wheel mounting surface and the centerline of the wheel. Different offsets can cause clearance issues with the suspension components or fenders.
* Center Bore Diameter: This is the diameter of the hole in the center of the wheel that fits over the hub. This also needs to be compatible.
* Wheel Diameter and Width: While not a direct fit issue, excessively large or wide wheels might rub against the suspension or fenders.
You'll need to find the specifications for both the 2004 Mustang wheels and the 1995 Thunderbird's wheel specs. Then compare them. If they don't perfectly match, the wheels won't fit correctly and safely. Using adapters is possible, but not recommended unless you are extremely knowledgeable about vehicle modification and the safety implications.