Whether you are installing Pushmatic breakers, Siemens QT circuit breakers or any other type, it's important to read the fine print on both the circuit breaker instructions and the breaker box. If you are not installing a specific type of circuit breaker in accordance with the instructions for the breaker box, you may risk voiding your warranty or creating an electrical hazard depending on the installation. Some circuit breaker boxes are designed for specific types of breakers or configurations of those circuit breakers.
If your Siemens circuit breakers are not right for the type of breaker box or if the breaker box is too full to accommodate a new breaker, do not perform "unauthorized" modifications to the breaker box or the Siemens breaker itself. Alterations to a breaker box or Siemens circuit breaker specifically provided for in the manual or instructions can be performed if you know what you are doing, otherwise consult with a professional to get suggestions on acceptable alternatives to your current setup.