We did a similar migration, from a model 390 to a F40 in the following
manner:
        1.  Install ADSM server on the new RS6K at the exact same PTF level
as on the old server.  Size the database, logs, and disk storage pools to at
match the size and names of those on the old server.  Given the difference
in AIX level, you will have to install a different client version.
        2.  Quiesce the old server, flush disk storage pools to tape, and
take a full database backup.
        3.  Restore the database backup onto the new server.
        4.  Tailor the new server from a library attachment and device
standpoint to match the physical configuration.
        5.  After it proves to be running OK, then consider migration on the
new platform.  Keys here are, don't change too many things at once and
having the old server available on the old platform gives you a backout.