If it's a completely stock 383, 10" is too low. You should be around 12" or a little higher. If the idle doesn't settle after higher rpms, you may have a vacuum leak somewhere or the carb throttle blades are hanging up. Next time you try it, try taping side to side with a very small hammer, on the carb throttle shaft(primary side),just to see if the idle drops back to normal. If it does then you know it's the carb, if it doesn't, then you know it's a vacuum leak or maybe timing. With 10", I'm leaning toward a vacuum leak, or retarded timing. 0* @TDC, with a stick car is normal with the vacuum line off of the distributor.