Water Wetter is usually good for about 10º F. We use it our desert race car in the heat of Nevada and Mexico, and it helps. Our class demands a STOCK cooling system, with no modifications, and we push 37" Tires through the silt at wide open throttle on a 2012 Toyota Landcruiser. It'll hit 235º when we're working it, but it holds together.
My $.02 on cooling in these old IDI's is that it usually comes down to the radiator. I've been leery of the chineseium aluminum radiators for a while now. They're cheap and shiny, but I'm not sure they flow at the correct rate. If they flow too fast, the coolant isn't in them long enough to cool off, and if they flow too slow, it backs up the flow and the engine can't shed the heat. I'm still running my original Ford radiator, and tow heavy in the summer moving race cars and parts round Vegas in 115ºF ambient temps. Mine stays in the normal range of the gauge, and I can hear the fan clutch lock up and unlock depending on load. Far as I can tell, my cooling system is working fine, and I've got an ATS intercooler and the AC condenser in front of the radiator, with the AC blasting in the summer. (I've noticed the AC isn't as great as it used to be- I think next summer will be time for some AC work...)
I've begun to plan on a radiator replacement, as mine has a small pinhole forming in the top tank. Given that it's 25 years old, a replacement is probably in order. I've contacted Griffin radiators about producing a proper aluminum replacement, which they swear they can do, but it won't be cheap. It'll likely be in the $700-$1000 department, similar to a Rodney Red. I'll also look into Ron Davis and other custom radiator manufacturers that we use in the desert racing/ultra 4 community. I'd encourage you guys with cooling problems to do the same. I don't think an auxiliary radiator is the solution, as it adds unnecessary complexity to the system. I've never seen electric fans that are sufficient for heavy towing, so I think the mechanical fan is the way to go. I will say there is some room for improvement on the shroud- sealing it properly against the radiator. Also, sealing the sides and the top of the condenser and IC with foam will direct the flow THROUGH these components, rather than around them.
Lastly, a proper ratio of distilled water to coolant is imperative for efficient heat exchanging. Nothing moves heat as efficiently as pure distilled water, but we must have antifreeze and SCA additive in the systems obviously. Those of you in a hotter climate, where it rarely freezes, can probably get away with a little less antifreeze and a little more water than those guys up North. My truck sees a variety of temps, from Northern Nevada in the winter to Mexico in July, so I run my mix protected to about 20ºF, and use the block heater when I go north, just in case. I realize this may not be feasible for everyone, but it helps for sure when practical. I like distilled water to keep the crud out of the cooling system, and I use a bottle of water wetter along with SCA additive in my truck.
All this reminds me I'm due for a coolant change. Perhaps I should get to ordering that new radiator and do it all at once...
Ryan