cross building and installing from i386 to amd64

As noted in the message on "more field reports..." I managed to boot
from acd0 in 7b2 i386 (finally got that cd burned by begging a friend
thus was able to install vista that requires it be the first partition
on the first boot device [not pata?!?!] thus had to uninstall
8-current amd64)... so I reinstalled 7b2 and now want to
crossbuild/install 8-current amd64 over it... how?    I use the corss
build procedure in build(7) and set DESTDIR=/ and it works find until
about 2/3's of the way through installworld (after installing the new
kernel but not rebooting).... on reboot init crashs.