I finally came across the entire letter from Ezra Stiles (President of Yale, 1778–1795) to Jacob Richardson, July 7, 1794. You can access that and another letter here. (I can access it through my institution.) You may be able to see a portion of the letter in Edmund Morgan's book that terms Stiles "A Gentle Puritan."
Stiles ironically doesn't come off so gentle in that letter. There Stiles, fervently supporting both, connects the American Revolution to the French Revolution and called for MORE use of the guillotine.
Stiles is interesting in that he was, as far as I can tell, a traditional orthodox Trinitarian Christian. The narrative with which we are familiar posits that it was enlightenment deists and unitarians -- many of whom understood themselves to be "Christians" as well (indeed, many of them ministers!) -- who posited the more cutting edge, controversial notions of the time.
With Stiles, though, it looks like we have a notable orthodox Trinitarian Christian who was like minded with Thomas Jefferson, Joseph Priestley, Richard Price, etc. I don't think he was alone in this regard (Samuel Miller of Princeton, for instance).
It's no wonder that Ben Franklin trusted Stiles with his religious secrets.