Got Shell 93 Octane and now barely idles 1973 Dart 340

The way you describe this, the engine is getting too much air, and/or not enough gas. Water in the gas accumulates in the bottom of the wells, and since that's where your idle pick-ups are, water passing through the idle circuit, at idle, will produce an instant stall. Of course the mixture screws will do nothing cuz more water or less water is not gonna solve it.
-So I'm gonna guess, the gas you got has oxygen attached to it. See, too much air/ not enough gas. Ima thinking you got a load of oxygenated gas.And screwing the idle mixture screws all in produced the stall cuz that circuit is working perfectly. Screwing them out produced no change, cuz they are at their limit of enrichment, internally.
-This happened to me once, Exact same symptoms. Since I was on a roadtrip I simply upsized the mains and cruised it all out. Next fill up I reinstalled the previous jets, and everything went back to normal..
-Pump it out, problem solved.
-I would go back to that station, and read the fine print, on the bowser.You may find the words "may contain up to 7 million percent alcohol." Or maybe a little less %! lol