No breakaway/down shift

If it shifts correct (firm and at the right speeds) the problem has to be in the valve body or TV adjustment. The valve body is the brain of the trans. that tells it what gear to be in. First thing I'd check is to make sure the carb is going to wide open throttle when you floor the gas pedal. If not you won't have kickdown. If it is in fact going wide open make sure the TV is adjusted so it's going to max at wide open throttle.

If the shifts are soft and quick it's possible the line pressure is low.