1973 318 hesitation?

If the RPM's increase but it's not accelerating then Maybe something's holding it back. Does it roll freely in neutral down a slight decline?? Torque converter or Brakes locking up could cause that. Or if the R's are increasing but nothing's holding it back then possibly the Transmission is slipping.

The key words in you original post are RPM's are increasing and that you hoped the Parking Brake wasn't engaging.