I meant try mixing the linux-image-amd64 kernel on a i686 build machine.
That should work.
In other words, just run sudo apt-get install linux-image-amd64 on
your build machine. Then reboot. Boot that kernel. Never mind if the
build machine is bare metal or a VM. No full 64 bit re-install required.
Just one package.
[Only if that does not work (unlikely) you must use a 64 bit Debian [or
derivatives].]
[[kernel architecture does not equal package architecture. Some can be
mixed. Multi arch.]]