If you reach up inside the dash from the driver's foot well, you will find a fairly good size plastic thumb screw (the part you grasp with you fingers is about 1/2" diameter) at each end of the cluster. As you start to unscrew them, the cluster will begin to fall away from the dash panel. Also, the speedo cable screws onto the cluster at the RH (inboard) end of the cluster. The first time you remove the cluster, you may need undo the two thumb screws and let the cluster drop part way into the dash board in order to familiarize yourself with the speedo cable location. If you can find it before you loosen the thumb screws, that's better yet. Once the thumb screws and cable are completely loose, you should be able to lay the cluster face-down inside the cavity and unplug the wire connections. Then you can pull the cluster out through the opening by rotating it slightly and taking one end first.

BTW - The thumb screws "should" have retainer rings that keep them intact with the cluster. If not, and they fall out, it's no big thing.

About your parts car (73) - You wouldn't have the original radio would you?