Thermostats - high flow vs standard flow - now I am confused

Well, I hate to sound like a broken record, but the flexfan has to go, and you gotta get a shroud.
At speeds above 30/35 ram air is sufficient, or at least doing most of the work, to make the cooling system work. But below that it's all on the fan.
I see two problems;
1) the lack of the shroud allows the fan to pull air around the tips, instead of through the rad and
2)Flex fans, no fan-clutch, 3.91s and a 4 speed is a recipe for disaster. Because the engine rpm is almost instantly up, and the flex fan does what flexfans do, stops pulling. But just as bad is that now, without a clutch, it becomes a road block to air trying to be rammed thru the rad by windspeed. I mean the air coming in only has 1" to turn and try and get around the spinning wall called a flex fan.
By 20 mph your rpm in first gear is up around 2600. By 30 then, it is 3900 so you have probably shifted into second and are cruising at 2800. Pop your hood and watch the fan flatten out as you rev the engine to these rpms.
With the engine idling, put an 8.5x11 sheet of paper in front of the rad. Notice how hard the paper is being sucked onto the rad. Now rev it up to 2600 and again pull the sheet off. Compare the two.

Ok now the solution to your problem is this;
1) a 7-blade all-steel, hi-attack, fan, and
2) a tight-fitting shroud, about 1" at the closest point, closer if you have restrained the engine, and
3) pull the fan away from the rad until the blades are half way out of the shroud. and
4) if you got the cash, I highly recommend a HD Thermostatic fan clutch.
The thermostat sets the minimum water temperature. In the same way, the T-clutch sets the maximum water temp........ so long as the rad is physically able to shed the heat. Once you have the engine operating between the minimum and the maximum, then you can tune the beast.
5) optional but very important, is to get fresh cold air down the carb.As long as it is sucking hot air, the engine will never reach it's full potential.

Here's something to think about, your fan is not just there to pull air thru the rad at speeds below 30/35 mph. In order for air to come into the engine bay, the air that is already there has to first make room for it......... so the fan also has to push it out. And it has to fight the hot air near the headers which is expanding and desperately trying to rise up thru the hood and is usually getting sucked into the carb; None of which is a good idea. That flex fan is killing your engine, get rid of it.
That's my opinion..... and my experience.