Sounds like your kickdown cable is not adjusted correctly....with a little accelerator action, it kicks the tranny up a gear for passing. It is a simple adjustment if that is indeed what is happening.
Make sure your vacuum line is attached to the modulator...it should lead from the intake manifold behind the carb down to the right side of the tranny into the modulator. Your modulator is adjustable too if you want softer or harder shift. There is a little tiny screw inside the hole where the line attaches.