I know the post is old, but this can be useful for someone.
You can install Windows X64 from an X86 system using an image that contains both architectures, x86 and x64 bit on install.wim file , or simply put install.wim from Windows X64 in a W7 X86 installer.
So you can install Windows X64 in the letter you want from 32-bit.
I hope it is useful for someone.
Viktor k.