What idle speed and mixture settings? Base timing? Sometimes, a hesitation can happen if the throttle plates are closed too much, which can alter the correct timing of when the transition port starts to handle the fuel supply transition between "base idle" and "run".
Also, check to make sure the accel pump is working.
At 25mph and under, with the stock 2.76 rear axle ratio, the trans should be at the bottom end of its speed range in "D"/3rd gear. Which would mean that a small amount of throttle to accelerate from 25mph and up would mean it was still in 3rd, with no automatic downshift into 2nd gear at that speed. As pre-'71 TFs did not part-throttle downshift for acceleration.
As the car slows below 25mph, it probably should do an automatic downshift into 2nd gear, though, just as it will automatically downshift into low gear as the car nears being stopped.
Youi might, for diagnostic purposes, see if it acts differently if you manually downshift into "2" below 25mph. To see if the accel response is better.
Does this happen when the engine is cold or after driving about 10 miles or more?
Just some thoughts,
CBODY67