I *think* I understand what you mean. The shock mount is in the shape of a U, and the new shock is too large to fit into the U - is that right?
Chances are the last time shocks were installed, the shock bolt was tightened too tight and compressed / closed the mount some. Take a pair of pliers / vice grips and open it up alittle.
If I've misunderstood then try posting a pic of what you mean.