I'm not sure just a thermostat is the answer. Finding the correct combination of things to improve cooling is tough, sometimes. On my 289 I tried many things to help it cool and some of my changes were to use less antifreeze. Great for winter, but in summer it really only adds rust inhibitors. So maybe a 25% mix may help. I used a bit more, about 35% mix, and a better-than-stock fan, a good radiator, added an external cooling fan in front, and kept the thermostat. Seems to be doing OK now, but Seattle is not Sacramento.
Consider making a timing change, as this can alter temps too. Make sure your mechanical advance and vacuum advance are working. Slightly more advance at lower speeds, but not enough to cause detonation under load, can help minimize overheating too.
Good luck! It can be a challenge.
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