All 1911’s need the thumb safety fit. The sear block needs to be trimmed down on a new install.
First test fit the thumb safety into the frame without sear, hammer or disconnector. Make sure the safety is moving freely while in the frame.
To fit the thumb safety install sear and disconnector and the hammer without the strut. Install the sear spring into the frame. Use the MSH to hold the spring into place but only slide it up about ½ way so you can see the sear. Slide the thumb safety in and you will see where it’s hitting the sear. Pull the safety out and remove just enough material of the tip for it to slide all the way into the frame