Here's what I've done so far:
Tightened the spring perch nuts. I got about 1/4 turn out of each of them, getting them to 100 ft/lbs. I didn't heat them up, as I didn't want to roast all the paint, but if I need to keep going, I might try that.
Changed the bolts holding the bottom of the steering column to the frame. The bolts looked fine, and the holes are fine, but I replaced them with full-shank bolts which fit pretty snugly,
Increased the tension on the (7 tooth) steering box. It was already pretty tight, but I got a little more out of it.
Checked/tightened the ends on the tie rod and drag link. They already seemed pretty good, but several of them could take a little more.
Reset the toe-in. It was rather too much, about 7/16". Backed off on the tie rod to get it down to about 1/8".
Check the preload on the wheel bearings.
At this point, everything feels reasonably tight. The weather's terrible here, so it will be a couple of days before I can take it out and try it.
If these measures don't fix it, I think my next step is to change out the steering column for a two-tooth. I should probably do that anyway, because the one I have isn't oil tight. I have a disassembled 2-tooth steering column that's in reasonable shape, except that the threads where the steering wheel goes on are trashed. So I'll order some parts to let me clean that up and reassemble.
If I still have trouble, the only thing I can think of left to do is install new kingpins to try to get rid of that last small amount of play.
I'll report back here once I can drive it around some.