300 HP will move an early A body along VERY well. Now, if you are wanting to turn low ET's on a drag strip, then that is another story. But it does nto sound like you want a fire breather, just something fun. An honest 225-250 HP will do well for you if that is the case.
I am not sure you have said what your ultimate goals are for the car...???? Just sporty cruising, or all out drag racing?