This is a variation of the 'Clean Drive' downgrade process I've described before in several threads
1) Disconnect your raid drives
2) Connect spare HDD to SATA port 1
3) Install DSM 6.x from fresh 1.02b loader, create admin user and server name matching live system, DONT create volume/raid. Update DSM to match your pre-beta version
4) Shutdown, reconnect raid drives to SATA ports 2-n
5) Boot, login, repair system partitions on raid, shutdown
6) Remove spare HDD and reconnect raid drives to SATA ports 1-n
7) Boot, repair packages etc
My additional steps would be to power down the server between drive swaps and update DSM on the clean drive to the version matching the one running before crash.