If this is a race only deal, you can go after more hp if you are willing to run a matching converter. If you want to run it up to 6400 rpm say, then you would probably be fastest with about a 5500 stall. Like has been said already, don't chase hp at the expense of torque unless you are willing to get the right converter stall and rear gear to match. Get the airflow up as much as possible as long as port velocity doesn't get hurt. If it were mine i would be checking on a solid rollercam with endurance lobes, hopefully there is a combo that can provide lift to use all the airflow you wind up with.