You missed my point. I never said that the system should be a x86_64 base, only the kernel. And *that* works just fine on i386 installations.
I've been running an i386 userspace system on an x86_64 kernel since 2005. Never had any issues with it.
Debian today now even supplies a 64bit kernel with its i386 distribution. Although, I think you still need to install the i386 kernel first, and then upgrade to the 64bit kernel after install. The package is: