I do not believe that you should need to change any code. Been a while since I've tested a Win64 build, but I think the Ant driven VC++ build completed without issue. Don't recall the experience building within Visual Studio.
Likely a project file issue. The .vcproj file in the release is generated from the Ant build script, not Visual Studio, so it might be missing some features. You could likely generate an empty project file in Visual Studio and then copy the guts of the .vcproj in the release and it would resolve the issue. _WIN32 is defined for all Windows builds including x86_64 and Itanium builds. _WIN64 is only defined for 64-bit builds. The only place you need to have _WIN64 is where the code differs between 64 and 32 bit Windows and in general that is usually places where the _WIN64 code would work with current 32-bit compilers but the 32-bit specific code will work with VC 6 and the like.