Hey, thanks for the good advice. Things went well and the engine is warming rapidly to +80deg C and then holding steady. It makes a big difference at the climate control.
I noted the failure mode of the thermostat was that the metal arch over the external part broke off. It definitely was not an OEM part. But now it is. I also replaced the expansion tank cap.
Larry, thanks for reminding me to strain the fluid while returning it to the system. It looked perfectly clean, but when I was done the the cloth was full of numerous chunks of crud.
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi