Got Shell 93 Octane and now barely idles 1973 Dart 340

Any oxygen you subtract and or any combustible solvent you might add, will make it run better.
Here's your proof;
Take the carb off dump that junk out. Put it back on, fill it up through the vent stack with fresh known to be proper gas.Since it's an AFB IDK how much the float bowls normally hold, so try 3 to 4 oz to start. Pinch off the supply line to prevent any more of that junk coming in. Have a 10 oz or so mustard bottle handy, half-filled with that same good gas. Dribble a half a teaspoon of it into the two carb bores.
Have the fire extinguisher handy.
Annnnd hit it. If the engine likes it, dribble a lil more gas into the vent stack, until it runs normally. If she doesn't like it, wait til she idles the bowls a lil emptier.
If there is no difference whatsoever, well, color me red, I'm stumped
Wait here's an even easier test;
Leave everything be. Start her up. As soon as she's running, pinch off the supply line. Then dribble a lil of the good stuff down the primaries.Easy does it. If she smooths out, that's your proof. When the idle speed starts dropping that would be a sign that the bowls are running dry, so pour some good stuff into the bowls through the vent stack, until the rpm returns.
If this works, you got no choice but to suck the junk outta the tank.
You did keep the fire extinguisher handy,right?