And I think I’m going to call it a win. The new disks are in and working. I’ve even got the regular snapshot script working pretty well.
Never did quite get the two new boot disks set up with identical partition sizes, but it doesn’t matter since I attached them both to the mirror (which was limited by the size of the old 80GB disks) first, and then detached the old disks. At that point it expanded up to the available size, which was the smallest partition on the two new drives. They differ by a MB or two out of 160GB, not important.