[suburban99]

A couple of things I have learned doing my 72 LQ4 swap with stock rad.
I had a leak in the near the casting marks of the original water pump so I bought a new one.. and a thermostat. Installed the new water pump and thermostat filled from the top rad hose and started motor. Plugged in my bosch handheld that will read live data and watched the temp. Temp gauge is stock with the stock sensor tapped into the water pump. Gauge temp was cold but handheld temp was 200...then 230...then 240...bottom rad hose was cool...thermostat was not opening.
Removed new thermostat and compared with old one. See pictures...
Put the old thermostat in refilled block and started. Temps climbed again then thermostat opened at around 235 on handheld and all the water from the rad disappeared...topped up rad and it's been fine since.

2 things I learned. ---get a handheld that reads live data, even a cheap one like mine. And all thermostats are not the same.