The car runs well overall, but it idles sort of roughly. This can often be caused by a leak in the intake manifold, especially at the joint between the manifold and the block. So I removed the manifolds and double-checked how well the intake and exhaust manifolds fitted up against the block. The fit had looked pretty good when I assembled everything a couple of years ago, but it was definitely not perfect. So I spend some time filing the manifold flanges until I could not detect any irregularity or gaps. Then I reinstalled everything, with a new gasket. It had absolutely no effect on the idle quality. Bummer.
I also tried changing the spark plugs. I had been using some old Presto-Lite #78 plugs, so I re-installed some Motorcraft TT10 plugs that had been in the car when I bought it. Again, no effect on the idle. Swapping the distributor cap also had no effect. Oh well, maybe the idle is as good as it gets. Anyway, the car drives fine, and I take it out a couple of times a week. I reached 500 miles of driving since last December, so I changed the oil.