I'm going to go a bit simpler... Had a similar throttle response issue. While internal engine or carb issues are common, did you check the throttle linkage at the pedal? Sounds a bit cheesy but I had a ruptured (blown out) plastic bushing on my accelerator pedal that prevented the car from going wide open throttle. It's a simple check... Have someone depress the gas pedal fully with the engine off and check the linkage on the carb... If she's got about 1/4 inch of play is say you found the issue, secondaries are not opening. This would cause the car to feel fine at highway speeds but lack on the low end... Just a thought