So I thought I would post an update.
I finally got it all back together last weekend. I had the freeze plugs installed leak-free a week before that, but when I reinstalled the head the tail of a zip-tie managed to get stuck between the head and the gasket, towards the back of the engine where it is difficult to see. Didn't notice it until I was trying to put the rearmost pushrod in, and something was blocking it. Removed the head and luckily nothing was damaged or cracked. So I had to wait for new headbolts and gasket to arrive. (tally for the entire operation: 3 head gaskets and two new sets of bolts).
Anyways, the sealant I used for the succesful plugs was Indian Head Gasket Shellac. I put a good smear of it around the bore and around the plug lip, and let it set up for about 20 minutes, until it is semi-hard. After driving it in as carefully and as straight as possible till it was just inside the edge of the bore (less than 1/16" usually), I let it sit for 24 hours before I filled it with coolant.
Besides the gasket shellac and letting it set up, the other important thing was driving them in straight and true. This is obvious, but with the engine in the car it is much easier said than done. There is not much room to swing a hammer, and the angles are awkward. I could not usually get enough oomph with my little plastic dead blow hammer, which I would rather have used. Instead I had to use a 2 lb sledge.
The key is to tap as gently as you can, and constantly observe if it's going in straight, and do correcting taps as soon as you see it off kilter. Usually it would start getting off slightly when I started it in, and then I had to use correcting taps the rest of the way. But if it get's too far off you really need to pull the plug out and trash it, since it will get deformed and will not seat as well, even if you can knock it back straight. If I had access to a hoist I would say that it's worth it to pull the motor to do this right.
Over the course of this I used several different manufacturer's plugs, including Dorman, Sealed Power, and Clevite (found the dorman at O'reillys, the others at NAPA). There were slight differences in cup depth, and even in diameter: the brass clevite plugs were a hair wider than the others, and you couldn't even start them into the bore without a hard whack, which usually got them in crooked. I had to throw all but one of those away. Most of what is in there now are Dormans, marked "2005" (there are other Dorman plugs that are labeled "CHINA", these have a shallower cup). But I had leaks with all types of plugs, so it's probably not an issue with a certain manufacturer.
I'm still not completely sure why so many leaked. It had to be some combination of wrong sealant, not letting the sealant set-up before driving them in, and not driving them in straight and to the proper depth.
Anyways, hopefully this can help someone in the future.
Now that I've written all this, one of the plugs will probably blow out on the freeway.
