Water plain will run the coolest temperatures, 100% Prestone is ineffective. Prestone's purpose is to keep water from freezing, a side benefit and small one is it will raise the boiling point of water somewhat. So if it were my car I use the proper ratio of Prestone and water and try again, maybe get a 180 or 190 stat too raise the warm up temp threshold. Possibly the 160 opens to soon and acts as if no stat were in the system.

Shroud - Speedway Motors, America's Oldest Speed Shop

As to the transmission an auxiliary cooler is easily plumed, also if you are running a stock stall converter you will more then likely be fine. You mentioned the shroud you have might cover some of the back side of the lower radiator tank. The air flow could just go around and actually provide some additional cooling. If you have room consider an electric fan many have shrouds built into the design, just a couple of ideas to chew on.

Building a fiberglass shroud, here a good article and reference.
http://www.crankshaftcoalition.com/w...ass_fan_shroud