Because its at the same rpm, could be jetting (metering rods on those carbs work off of vacuum) or a vacuum problem (vacuum advance??) Could be something as simple as a "flat spot" in the carb causing the engine to flutter around. If you have a different carb to try, swap out in 10 minutes and see what it does. How about the vacuum advance??? Simply plug the vacuum advance hose and try it. Too often the simple and obvious can be overlooked. Never know when you buy a car the adjustments made by the previous owner...... but I would think a flat spot in the carb would be a prime suspect. Loose torque converter bolts are noisy but usually don't cause a viberation. Touching exhaust would normally find another time to touch other than that particular rpm range consistently. Usually these things aren't as deep as they seem.... usually.
