Simple "fix" ...... you have a new thermostat, so remove it, dunk it in hot water and make sure it is actually opening. Just because it's new doesn't mean it works. If it isn't working, the lower radiator hose will be "cold." Hopefully the "fix" is as simple as a bad thermostat.
Something else to observe. With the engine cold, remove the radiator cap. Start the engine. When the engine gets to operating temp, and if the radiator isn't 100% plugged, you'll see & feel hot coolant coming into the top of the radiator as it flows in from the head.
Theory: with the lower hose being cold, there's no flow through the radiator. Thus by removing the radiator cap, you'll now get flow up and out of the radiator fill spout since the spout becomes an exit location as the water pump pushes it. This is also a check on the water pump to see if it works.
Depending on the above, the cold lower radiator hose tells me the radiator itself is "sufficiently" blocked and is the problem (this assumes the thermostat is OK).
My thinking tells me the water pump is working to some level to circulate coolant because if coolant wasn't circulating at all, coolant just "sits there" and very quickly gets to boiling and beyond. And thus being a closed system (i.e., coolant circulating only in block and head), it reaches some sort of equilibrium at a very high temp, in this case too high for safe driving.
My thought: remove the radiator and backflush to check "blockage factor:" garden hose taped/sealed into the lower outlet and full-blast with the hose, with radiator fill spout pointing at the ground. You'll know soon enough what the blockage level is (if any).
In my '74tii I use the original radiator design, and years ago I found a 3-row core (original issue is 2-row). A local radiator shop installed it using the original upper & lower tanks. After that, no more heating concerns with 3 rows.
SO, maybe you can find a "bigger" radiator with more rows ??