What people call
pgp5/pgp6 seem to come from
http://www.pgpi.org/, they're largely unmaintained and quite abandoned, or should be abandoned rather.
If the FreeBSD port is marked for i386 only, it's possible that they simply won't compile on amd64.. maybe it's not 64-bit safe?
Most people use the GNU projects GnuPG, which is OpenPGP compliant.. but indeed many do use one of the other commercial/unmaintained products instead.
Perhaps it might be possible to use GnuPG, or if FreeBSD supports it, run the i386 executables on your AMD64 server.. if all else fails, try compiling the available source code yourself.