I have v6 x64 and I was just trying a few things out, It seems that with the v6 SDK i no longer get linker errors at least for the FTK library.
Its a bit funny i am compiling against both 5.1 and 6 just to get it to work because v6+ doesnt seem to have the Core library (XSICore_static.lib) so I need to investigate, they probably merged it into sicoresdk.lib
So i guess no need for you to go hunting for 5.11 unless you feel compelled

but if you do I would still appreciate it <3
No worries though if you cant find it.
Can you tell me what FTK stands for? File Transfer Kit?
I'll probably make a new thread for a 64bit version of the valvesource addon once i finish compiling and testing