Thermostat choice? What temperature do you want your engine to run at? The thermostat sets the maximum operating temperature of the system. That's assuming the REST of your system is up to the task of removing enough heat. So if your radiator, and water pump are clogged, or worn out respectively, or your ignition timing is retarded, or your engine cooling system surfaces are corroded, no thermostat can do it's job. If everything is right, the system should not run at more than 5-10* ABOVE the thermostat rating. The radiator is probably the most important component. Get the biggest capacity radiator you can, 2 row, 1 1/4" tube aluminum. Then the thermostat will open and close to maintain it's rated temperature. The rated temperature is the temperature when the thermostat BEGINS to open. It will not be FULLY OPEN UNTIL 20* above that rated temperature.
Since you are already running a 180* thermostat get the hi-flow unit. Way more water can flow through them. You would want a hi-flow thermostat anyway with a hi-flow TA water pump otherwise it would be restricted. A 2 core aluminum radiator should be plenty of cooling for a 350 going down the highway, even a 455. Normally a car with decent cooling flow should run right down at the thermostat temp going down the road at part throttle, 50 mph on a cool 50-60* night. If it doesn't that to me means flow through the system is iffy. On a cool night it barely needs a fan at easy cruise. A 2 x 1" core aluminum radiator should be plenty of cooling for a 350 going down the highway or just about any situation. Maybe at idle with AC on it might get dicey. No way it should get so hot cruising even on hot days. How new is that fan clutch and what brand? P/N? When you start the car after it is cold the fan clutch should blow alot of air through it as you rev it a bit. My RMW takes a mile or two before it loosens up. Sounds like a storm under there until it does. That is entirely normal as the fluid in the clutch has to re-circulate for a couple of minutes until it releases the fan. After that it should loosen up until the temp gets hot. If it truely is working as you rev it you should be getting a whoosh of air just like it did when cold. A real blast of hot air if you have your hand there just like when it was cold. One of the Severe Duty fan clutches may help the situation before springing for the TA pump since you say it runs even hotter at idle when air flow is poorer. On hot days with AC cruising down the highway it would even help. Forget HD clutches and go right for the Severe, it is the only fan clutch I run when I really have to cool an engine. https://www.summitracing.com/parts/hda-2747/make/buick Something else to consider, is there any cylinder pressure leaking into the cooling system? It wouldn't be much but it would add heat to the system. I'd do a cylinder leakdown or radiator pressure test. As mentioned make sure there is good flow under the radiator cap once it is running and hot. Check for bubbles. What converter? What gears? Performance setups could cause an engine to run hotter.
Many cars of yore not only had marginal radiators sizes, they did not even have the fan positioned the right way to draw in the most air, that being having the fan blades half in and half out of the shroud. If the motor can run at fully warmed up idle for 8 minutes at no more then the limit of 10 degrees above that rating of the T-stat, but exceed that temp at part throttle ( 50 plus mph ) going down the road then you need a bigger radiator assuming no flow restrictions are taking place from a collapsed lower or a crappy ribbied type upper or lower hose.
