I'm not gonna lie I'm a little confused as to why using a different CPU would cause your graphics card not to fit?? I've never heard of this problem.
Theres only one form fitted spot to drop in the CPU; any CPU of the same socket will not have a larger/smaller diameter than the other.
You have no choice but to use a CPU from the same socket as the CPU currently in your motherboard. You can't use an INTEL cpu in an AMD motherboard or Vice Versa.
Other than that you need to give a little more information because right now the ONLY situation I can think of that you would run into this problem is if your attempting to get a CPU bigger than the socket on your motherboard which won't work Lol.