It is not impossible to do it, but you have obviously got the wrong sort of MCBs that don't fit. Take them back and get a refund if you can
As the CU sounds to be very old, you are likely to have other shortfalls that mean it is ripe for replacemnt. The cost of having it inspected, and the circuits tested, and brought up to scatch, to get the job done properly, means that it is unlikely to be worth it. It is notifiable work that has to be done by a suitably qualified electrician, or inspected and tested by your local authority at additional cost.
The old type of CU is still made, and in a few cases is on quite a modern installation, which may be worth converting to MCBs. If you post a pic of the CU and the wiring around and between them, and tell us the age of your house, we might be able to tell you more.