1-Download source code from (http://trac.osgeo.org/proj/)
At the time of writing this post 4.8.0 was latest (http://download.osgeo.org/proj/proj-4.8.0.tar.gz)
2-Extract all files to c:\proj
(this is important to follow this path or you need to change this file path in make files)
3-find the x64 nmake path on your system, you can use "where" command it will list all nmake versions that you have on your machine. make sure you are using x64 command prompt or it will list only x86 nmake files.
Even if you are unable to find the x64 command prompt x64 version of nmake resides inside amd64 folder on the same path. Following are example path for nmake.
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\nmake.exe
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64\nmake.exe
4-Once you find the path of x64 verion of nmake, first you need to prepare environment variables.
Execute the following command
c:\proj\src>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64\vcvars64.bat5-Build the source
C:\proj\src>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64\nmake.exe /f makefile.vc6-Find the 64 bit dll at "C:\proj\src\proj.dll"
You can also download the 64 bit version of this file from here. proj.dll x64