I found this discussion on compiling for "Any CPU" vs. 64/32 bit:
It seems I can get away with offering 1 version. I just have to detect the system ingame so I can restrict the large maps to 64 bit systems and thereby prevent out of memory crashes.
As long as all of your DLLs are also 'Any CPU' it will work. If any of your DLLs are 32-bit, you'll crash if your EXE launches in 64-bit mode (and vice versa). You'll get a BadImageFormatException when the process discovers that it can't load the DLL.
There might be a way to provide alternate DLLs per architecture using the manifest, but I haven't tried that.