I'll add this.
160 F is cold for normal operation. If accurate it means the oil (which tracks similar to coolant temps during normal driving) probably isn't into its operating temperature range. Downside of cold oil temps is moisture isn't boiled out and the viscosity is higher than it should be for best flow.
210 F is not fine as long as it doesn't continue to climb.
The differences at idle vs while driving have already been covered. So it does not seem to be due low pressure (bad cap etc) but probably idle air flow or less likely, pulley and pump ratio.
Last possibility, and one that has not been mentioned, is timing. On CAP and CAS era cars the initial timing was reduced compared to earlier engines. This was done to continue the combustion further down the cylinder. Doing so made a more complete burn and put more heat into the cylinder walls - which helped the subsequent burn. Other things were done to somewhat offset the various drawbacks of late timing at idle. No idea what is on your car now much less what it began life with. But its something to keep in mind.
If there are no emmissions (other than PCV and fuel vapor capture) a 340 should idle nicely around 650 rpm at 10 to 14* BTC.
With early emissions it was more common to see 700 rpm (or more!) at 0 to 5* with a real fast advance above idle speed.
