Install pkg_comp and read the man page (very well!).
You will need to create a chroot with sets from the older version.
This requires editing the pkg_comp config file (after making a
template) to point to 1.6.2 sets, and probably changing the set names;
I found that pkg_comp was set up for 2.0/current and I think X the set
of sets was slightly different.

In the chroot (do 'pkg_comp chroot' to get there), it seems you should
install pkgtools/libkver.  Then you can build packages.  Because you
are in the chroot, the 1.6.2 compiler (installed from sets) will be
used.

It is key that you have a 'newer' kernel, where newer really means
that it provides all syscalls expected by the bits in the chroot.
So your 1.6Z kernel needs COMPAT_16.

-- 
        Greg Troxel <gdt@ir.bbn.com>