Big Block Dart Cooling

Do the easy stuff first. Back timing down 2* at a time and see if it helps. If that doesn't work try a clutch fan. Be aware that crank driven fans don't function above 35-40 mph. Next, pull a spark plug or two to see if engine is lean. Could need to flush cooling system. Maybe the rad is not up to the task. Lastly, I would purchase a direct reading temp gauge. Moon equipment sells them through " Speedway", liquid filled, $35.00 mount on the water pump housing to see what's happening at the outlet. This happens all the time with A-bodies BB. Stick with it, you can solve this. Good luck, Tom