Several things come to mind as possibilities that you haven't mentioned, in no particular order...but I'd REALLY focus on my IR gun comments, and your shrouding first. Then maybe t-stat and water pump comments next...:

Fan shroud. You say you have electric fan. What type? Is is FULLY and COMPLETELY shrouded so that 100% of the air it pulls is coming THROUGH 100% of the SQUARE surface area of the radiator, and not just at the center of the radiator in the circular shape of the fan? A photo or more complete description of your fan setup may be helpful...if not properly shrouded, 2/3 of that big radiator may not even be utilized...

Have you done any testing with an IR gun? There is no better tool for tracking a problem like this. Take some readings on your radiator when the temp starts running away on ya. This will help you narrow down the problem. Is the radiator as hot as your gauge says? If yes, this eliminates restriction possibility in flow, and leads me to back to inadequate airflow or some other reason for the radiator not transferring heat to the air. If temp at the radiator is much lower than gauge reads, then your theory of flow restriction may hold true.

With that IR gun, make sure you compare inlet temp at radiator to outlet temp at radiator. This gives you another huge clue as to whether or not the radiator is actually dispersing heat like it should. If you see similar temps in-out with little drop, it could be the result of poor shrouding/airflow, OR improper coolant providing poor heat transfer, OR coolant flowing too fast thru the radiator to dissipate heat before it exits.

"new water pump." What kind do you have? Assuming stock, you haven't said anything about checking diverter plate-to-pump clearance at time of installation. Almost every Pontiac-style waterpump needs this set closer than it is stock, in order to pump coolant efficiently. A big gap leads to very poor coolant circulation. You want a TIGHT gap. http://www.wallaceracing.com/water-pump-mods.php

Is your water pump equipped with a stamped steel impeller, or a cast impeller? Normally a cast impeller is superior to stamped and will pump more water better.

COUNTERPOINT to not enough flow from pump: TOO MUCH coolant flow can indeed be a BAD thing. But even the MOST powerful pump in the world should not be able to overcome a proper t-stat restriction. Your say you have a high flow 160 t-stat. This is not always a good idea and might contribute to your problem. It might be flowing TOO FAST and TOO EARLY, and not re-closing/restricting flow enough for the radiator to have time to transfer heat to the air properly. Sometimes a standard flow t-stat, in maybe a higher (180?) degree reading, will slow the coolant flow down enough thru the radiator that it actually has time to cool before exiting the rad and re-entering the engine. Inexpensive/easy experiment. T-stats NEED to cycle...open/closed/open/closed/open/closed. If yours is just staying open and flowing like mad because it is rated too cold and your pump is too efficient, this can indeed cause an overheat situation like you describe.

What mixture of coolant did you install? You want 50/50. Some think stronger is better. This is completely wrong. Anything stronger than 50/50 that has MUCH lower cooling capacity than a stronger mix because ironcally Anti-freeze is TERRIBLE at cooling. WATER is the absolute necessary ingredient in the mix for proper heat transfer.

Do your hoses have proper support springs internally to prevent collapsing? Have you visually checked your hoses during oveheating problem to see if they're still round and not flattening out?

How did you set up your carb? If it's too lean, it'll run too hot. MANY things can cause lean running, and not just the carb setup alone. Carb-to-intake manifold air leakage is an extremely popular and often overlooked problem, as are most other vacuum leaks. Use a propane tester and search for vacuum leaks all over.

You say your timing is "right on". How did you determine this? Factory setting? Chassis dyno-tune? Recommendation from Comp cams? Some of the mods you mention may result in a need for different ignition timing. How did you determine proper timing for your new setup?

No promises obviously that any of these are actual solutions, but hopefully it helps you think it thru and gives you another set of eyes...wish you the best in finding a solution.