Are you trying to make a 64-bit Linux system starting with a 32-bit one?
If so, you will need a 64-bit toolchain (linker, assembler, compiler) and 64-bit 'C' libraries. This is why most start out with a 64-bit distro (unless you go the Gentoo route and build a 64-bit system from source).
It's funny you ask this question since I'm pondering doing this exact some thing, building a 64-bit Linux system from source, starting with a 32-bit distro.
EDIT; I just found this
. It's not Slackware specific but it should be applicable, nonetheless.