Oracle Database Discussions

Hi All,
We are using Oracle 10.2.0.4 on IBM AIX5.3 64-bit.
Our database has reached size of 40TB. Now we are revising our backup strategy as the current option is having constraints.

We were taking Weekly level 0 backup and daily level 1 backups but now the problem is that 40TB database will take more than 24 hours for complete backup.
We have 4TB file system where we can place our backup to speed up the backup time.
Similarly we have 40TB TAPE storage for backup to be stored permanently.

I have placed the database in archive log mode so that we may utilize the block_change_tracking feature for incremental backup.
We have most of the tables with Range-partitions Which can help us making the old partitions tablespaces as read-only and skipped them during future backups.

Business requirement is that the critical data must be available within 2-4 hours of disaster, the medium priority data should be available in 24-48 hours and the history data can be make available in 5-6 days.

Is it recommended that we divide the base backup into two sections. i.e. take one level 0 backup of database by including history data only (data for 2007 and 2008 years)
and then take another level zero backup of database by excluding the history data(2007-2008) and include the remaining data (2009 onward till date) and take daily incremental backups.

I have practiced the EXCLUDE_TABLESPACE method while taking backup and have restored the database and it was successful, Similarly I have tested the RMAN backup by skipping the READ-ONLY tablespaces and it was also successful. I want to confirm if the divide and rule strategy above is fine or there are any risks involve in this?

Can you please help me how to define a backup strategy for our environment.
What are the best practices for VLDB's ?
I have read the oracle documentation but need little more help on this:
http://download.oracle.com/docs/cd/B28359_01/server.111/b32024/vldb_backup.htm

Waiting for kind reply
Regards
Fkhalid..