360 performance help

Lack of fuel would be my first place to check, sounds like it's starving. You don't mention any misfire, so that should rule out valve float, ignition timing could be, but I'd start with fuel, low pressure, plugged filter, low float level. In my experience 6500 is pretty high for a stock 360, has the engine been built for higher rpm? Mine previous one would fall flat after about 5500, partly due to cam profile and cylinder heads.