You can do that as you install the OS or post install but it is much easier if you do it as you install.
I would recommend to make all drives part of the array and that way you'll have more than 1 terrabyte of space. Make the 73GB drives a /dev/md0 (raid1) and the (4) 300GB drives a /dev/md1 (raid5); then you'll have 973GB of space to play with.
After you set the raids then use LVM to partition your logical volumes using all 6 drives. If you use 10GB for all your system directories, which should be enough space, then you'll have over 950GB of space for your data; that's almost 1 terrabyte.