Bullet comes with both 32 bit and 64 bit configurations for every platform.
If you use CMake to generate your Visual Studio projects, simply select the 64bit version of visual studio, using cmake-gui (the graphical user interface of CMake)
Here is a screenshot of the 64bit compiler selection:
cmake_64bit.png [ 70.82 KiB | Viewed 667 times ]
If you use the premake to generatio Visual Studio projects, you can also select 32bit or 64bit inside Visual Studio.
Here are the steps:
1) Download Bullet 2.81 or later
2) Click on bullet\build\vs2010.bat
3) Open bullet\build\vs2010\C:\develop\bullet_latest\build\vs2010\0BulletSolution.sln
4) Select 32bit or 64bit, see screenshot:
x64.png [ 43.18 KiB | Viewed 668 times ]
In other words, there is no need to "port" anything. I'll add it to the user manual.