Twice lucky
Retorqued the head bolts (along with the exhaust manifold nuts, oil pan bolts, and a few other things), adjusted the valves, changed the oil, and she fired right up, running nice and smoothly.
Learned that the valves were pretty far out of whack (all of them were too open, the intakes more so than the exhausts, surprisingly), and most importantly (this is a lesson for all of us), the head bolts had loosened up a fair amount in only 300 miles of driving since replacing the head gasket.
Also, a subtle thing, the previous valve adjustment I had done used the "2-step shortcut" method I had read about somewhere, but this time I used the Fiat factory manual method (putting one set of valves "at balance" while adjusting another pair, etc.) and I suspect this is more accurate than the shortcut method.
Most importantly, listen to Obert's advice -- re-torque the head bolts 300-500 miles after replacing the head gasket, and again after maybe 3K miles (I will certainly do that).