Break the mirror. Installing on a RAID is not supported. You could if you didn't try with 10.6.3 DVD but then Lion and ML will not support install to an array. The Recovery Mode partition cannot be created on a mirror or any array.
You can clone the system when you are done. CCC does a great job. And you want to have another boot drive with the OS and with a recovery partition.
There are changes to the partition maps and arrays that can only be done by destroying the old mirror.
You never should use any volume w/o backups.
instead of installing over the old OS, do a clean install to a new drive.
you need backups for your mirror.
a better use is to have a clone and not a mirror. Mirror comes after backups.
Clone plus Time Machine and then mirror.
for a nice system drive, $100 Samsung 840 SSD.
For your data, 1TB WD Black, or use a mirror of two (aimfor 50% free).