Yep, Water Wetter, or a similar product, will lower your temp about 10 degrees. I've been touting this stuff for years now. However, the main problem is your fan. Run the stock clutch fan, or an HD thermal clutch unit, and a stock fan shroud.

When in doubt, stick with what the factory did. Pontiac had brilliant engineers that created parts that work for 100,000 miles or more, and their goal was make these engines run efficiently under extreme heat and extreme cold, and to keep GM from having warranty problems. They knew what they were doing.

Another thing I would recommend would be a change to a later style closed radiator system, like those on 1971-up Pontiacs. You need an overflow bottle, and a 1971-up radiator cap. This setup adds additional cooling capacity, allows you to run less pressure (always a good thing, less stress on your hoses), and prevents your system from leaving those embarrassing green puddles when you park it at a cruise night.