Visual Studio is x86 until at least the 2010 release comes around update: this is still an issue in VS2010, there is no native 64bit Cassini support.
Two ideas:
You could try to compile a 64-bit Cassini from source.
Even you are using 64-bit environment, temporary refer 32-bit dlls in Visual studio (or manual copy it in BIN folder) so that you can debug it. Keep in mind, every time you compile the code it will re-copy the 64-bit assemblies in BIN folder.
Use IIS on your local machine.