'92 Buick Roadmaster runs cold

Recently, I had my (otherwise fine running) 1992 Buick Roadmaster's intake manifold gasket replaced, because it was consuming coolant at this
location. With 282,000 miles on the vehicle, I had yet to experience a system failure worse than an alternator.
The problem is that, since the repair, the vehicle has run VERY cool. I replaced the thermostat with a 65359 (I think) Stant 'superstat' from NAPA. 195 degree OEM etc... After burping the system, the temperature rarely got above 160 (the temp gauge is actually quite accurate). I thought I had gotten a bad one (rare), so I replaced it. No change.
Only with ambient temperatures above 40F *and* 2/3 of the radiator blocked off do I get heat out of the heater much more than ambient. Today, with temperatures between 5F and 20F, I get barely enough temperature to defog the windows, and the temp. gauge reads between 150 and 160F.
What could be wrong? How can I diagnose this?
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