A 64-bit version of Chromium can be built on Linux by using the -Dtarget_arch=x64 flag on the GYP build system. (This is the default on 64-bit Linux systems.)
Neither Chromium nor V8 has a 64-bit version on the Windows platform right now. However, Chrome does run on 64-bit Windows as a 32-bit application. V8 should only need a small number of changes to build on the Windows platform.