Self learned car guy with limited knowledge here so bear with me. Prepare for the word vomit.
I live in Iowa so this time of year a functioning heating and radiator cooling system is very important. 37 degrees this morning, 76 at noon. I purchased this vehicle from California, and by and large it's a good vehicle. 510 4-door automatic 1972(hoping at some point to put in a manual). No engine mods as far as I know.
I drove it a few times over the summer on trips of 30 minutes or so. Each time I would drive the engine temp would slowly creep up to almost full hot. The heater core was deleted when I got it. I would reach my destination just in time and set there for a few hours before making my trip back home.
If I squeezed the top hose a few times before startup, the car stays cooler longer, I will be able to drive an hour uninterrupted before the temp gets too hot. If I stay at 55mph, my temp will not raise, anything higher and it rises. Transmission cooler hoses leak when under pressure, level looks good on the transmission though.
Got a new rebuilt carb and adjusted (made sure not too much air flow), flushed the radiator 2x. Took radiator out/flushed it, replaced water pump, flushed the engine (water flowed). No little bubbles in the coolant, oil doesn't change color.
A lot of rust and lime was inside the radiator when I got it out, but temperature readings on lower and upper hoses seemed to show the cooling was happening. If it is 180-200 on top hose, 120-130 on bottom hose.
Replaced thermostat, took thermostat out to check for maybe a blockage behind the thermostat, same result. Re added the heater core recently.
My test trips have a few stops in the country, 4 miles to the 1st stop the car will run just below the "E" in TEMP, no matter what speed I go. As soon as I slow down and speed back up the temp starts rising past the P, when I stop to check the temp with a laser thermometer the temp is around 200-215 or so on top hose. And engine block is warmer, lower hose may creep up to 140, but that is it.
Since adding the heater core, my trip to the location will have heat for defrost on the way up, but on the way back the heater is cold. When idling the temperature is good, in fact if it gets warm I can stop at a corner and keep it there for a few minutes at idle and it will cool off.
I'm totally lost, any advice?