Thermostat

The stat sets the minimum running temp. In winter if the heater core cant supply enough heat, we raise the minimum running temp with a higher rated stat.
In summer, we want to maintain engine efficiency, so I still run a hot stat. If you have over-heating issues, a cooler stat will not solve your problem, unless the stat is faulty. Remember the stat sets the MINIMUM running temp.
Engine efficiency increases with rising running temp, right until it cokes the oil or seizes a skirt. Modern vehicles are running "normal" temps of 220*, plus. Oil temps over 250* are not recommended. If your engine is set up for it, the sweet spot seems to be 220 to 230. But you better have a well planned cooling system, capable of shedding a lot of heat, cuz theres not a lot of headroom from 230 to 250.
Getting back to your 350.
If you're gonna run up into the mountains, I'd kinda suggest a 160/170.This will give you a couple of minutes to watch the temp gauge when climbing long grades, and verifying the cooling system is capable.
Down on the flatland, the 190 would be my choice.
Remember overheating is not a stat problem unless its not opening correctly.