Bolt Pattern 1966 200 Straight Six

grab a measuring tape and measure the distance from a lug to the opposite, center to center

American Racing's website lists 4 lug mustangs at 4 X 4.5.

Be cautious with adding aftermarket rims to a 4 lug 'stang. It can be done but I had a problem with my first mustang that ruined a complete set of drums. When you pull off the rim, you will see a weight that protrudes out from the face of the drum. This weight will prevent a flat back rim from seating flatly against the drum face. In my younger days, I found a cheap set of used chrome dish rims with a bolt pattern to match my mustangs. I raced home, slapped em on, and drove around the block a couple times to test them out and show them off. Even with as little as I drove it, enough heat and force was generated at the drum that it warped all four of them. Brakes locked up (most likely binded) and I couldn't move.

I replaced all four drums and found a shop that some how was able to balance the wheels on the car (have no idea how) and everything was fine.

This is not an issue with 5 lug cars because the face of the drum that meets the back of the rim is raised from where the drum weight is, and it's not an issue with factory steel rims on 4 lug cars because the back of the stamped steel rim rolls out just past the lugs.