If you do have 6 x 70 Gb scsi drives, they pretty much have to be on a controller. You could use linux software to setup the raid 0 + 1 setup, but the controller will probably do hardware raid, which is much much better. There is probably a way to get to the controller during boot, usually before you would get into normal BIOS.
In any case, try to get into the controller, setup two 3 disk groups, then setup the raid 1 mirror between them. Then when you go to install, it will only see the 210 Gb of space, probably as /dev/sda.
With that number of disks, you may want to look into RAID 5 or 6 too. RAID 1 is very fast, but you loose so much space to parity. Also something to check out, is that 0+1 is not 1+0, ie stripes of mirrors are not the same as mirrors of stripes. They are close, but do behave different.