A few points:
If the engine gets hot at highway speed, your radiator is inefficient. Plain and simple. At 30 MPH you have more air coming through the radiator than the fan will ever move, shroud or no shroud. If turning the heater on brings the temp down, this confirms an inefficient radiator.
You are running a V-8 radiator right?
The thermostat only controls the minimum operating temp of the engine, not maximum (but with an efficient radiator, the spread should be slight). Here's a quick way to verify thermostat operation and gauge function: Start cold engine and let idle. As engine comes up to temp, keep feeling the top hose. At some point it will get a lot warmer real quick. That's when your thermostat opened. Look at your gauge and note reading. If you have a 180 thermostat, that's 180 on your gauge (should be mid scale). If the gauge continues to rise at idle, the radiator isn't doing it's job.
Teflon tape on the sender would cause a low reading if anything. But the dash gauge may be inaccurate. Is your gas gauge optimistic or pessimistic? If the gas gauge tends to read high, the temp gauge may read high as well. This would be a function of the voltage regulator on the back of your instrument clusters. These were known to be inconsistent.
Radiator caps control the point where the coolant boils. If you are having boil over problems, replace the cap. If it heats up without boiling over, your cap is probably fine. Replace it anyhow, now your have a spare.
I think you answered your own question already. You said you got rusty crud out of your radiator once. I bet you need to flush more. Get some radiator flush stuff from the parts store and give it a shot. Or take the radiator into a radiator shop and have it cleaned properly. Sadly, in the modern age of aluminum radiators (which cant be repaired) radiator shops have become a vanishing asset.
When was the water pump last replaced?
