mahlon suggested, I set up the 140gb as a RAID of one disk, the copied the data off the individual 70s onto it, then added the 70s as a plex and let it rebuild. I think it may have been a mistake to add both 70s in a row, because I think they were reading different parts of the disk as they rebuilt, so it took a lot of time... but it was 140gb to copy, after all. Perhaps if I'd brought up their plex instead of doing them individually; I can't remember what exactly I did.The only scary bit is that vinum gets confused when you delete things, often reporting that you have -1 disks instead of 0. But other than that: painless.
Everything* I've wanted to install so far has been in the pkg_add repositories. I'm a little nervous about what happens when it's time to upgrade, but I also don't expect to upgrade much.
Also, I want to plug dnsmasq. It does DNS caching/forwarding along with serving names from /etc/hosts (along with many other things), which means I can put all of my internal hosts' names and IPs in /etc/hosts and everything just works. No more futzing with BIND and maintaining both forward and reverse records.
*(Except for edna -- it turns out that Greg Stein, who wrote this along with being director of the Apache foundation, etc., is a coworker and manager of the Blogger team.)