Lighter pistons...rebalancing?

i had a 383 in a car, i wanted a 440 so i rebuilt a cast crank 440 using the same crank, rods but changed the pistons to a set of kb (much lighter than stock) that had been used before and were all balanced (just the pistons) to the same wt. when i fired up that motor i could feel that it wasnt as smooth as the 383 was the 440 did have a 274 cam in it so it was going to be a little rough idling anyway. none the less it ran good and i never had a problem w it and turned it to 5500 many times till i eventually sold the car.