It shouldn't get to 3/4 on the temp gauge, that's at 100 deg C!
The steel pipes into the SU mainfold are always blocked, the fittings in the manifold corrode unless they've been replaced as part of a refresh. The flow through that manifold will be zero, that means no flow through housing past the thermostat and poor temp control. The bypass hose off the housing is a question mark shape, you'll never get a screwdriver down that.
There's most of the shite temp control. That's why the 'jiggler' is removed and the hole is enlarged. Drill it to 2.5-3mm, it's very a common on the L4-L6 engines.
Next time when filling the cooling system, leave the car on stands/jack at full height while filling, start the car and squeeze the lower hose when running to push any air up to the radiator, leave the cap off and you will see it bubble in the neck. Then repeat.