Big Block Dart Cooling

If your temp is climbing at speed, You have at least one of three issues; 1) the rad cannot get rid of the heat fast enough, and 2) your engine is generating more heat than the rad knows what to do with, and 3) the coolant itself is unable to affect the heat transfer to the rad material, and from thence to the airflo.
As to #1) make sure that all of the rad's surface is exposed to the incoming airflo.Or at least as much as possible.Also force that 60mph wind to all go thru the rad, and not around. Also make sure it can pass freely through the rad and over the fins, and out the otherside to exit the underhood area.If you have an auto with a hi-stall, you need an auxilliary cooler, to take the load off the main rad.Make sure the rad is physically big enough. A 26 incher works for me, at over 400hp. I wouldn't think of a 22
As to #2) The 3.91s are cranking up the Rs which is putting a lot of heat into the system. When the 3.55s go in, there will be about a 10% reduction in heat due to internal friction.
Then since there will be only 90% as many explosions per mile, that adds up to another 10 % less fuel being burned in the chambers, which adds up to that much additional heat not being generated, in the first place.
As to #3) Water is a far superior coolant than antifreeze.Since your cooling system is very nearly adequate now, I would simply switch to 100% water (with an additive pkg), at least until the 3.55s go in.After the 3.55s are in you can re-access the situation.

Additionally;
You say you are running a fixed fan.But make no mention of the type or number of blades.
If you also have issues from idle to about 30 mph, then,IMO, you might benefit from a large diameter,all-steel,7-blade,OEM type fan, mounted on a Thermostatic clutch, and situated correctly in the shroud. This set-up moves tons of air. This will eliminate heat issues below about 30mph.As soon as air-speed comes up from the car moving through it,and the exit temp falls, the fan will begin to slip, and eventually freewheel.Whatever you choose I highly recommend to stay away from small diameter fans with flexible riveted-on blades.
Once you get your heat issues under control, I have found that the hotter the engine runs, the more everything you get. More power,more mpg,more torque,longer cylinder wall life. I stopped at a minimum engine temp of 205*. That leaves a little wiggle room, for emergency situations.I don't know where the benifits stop. My little 367 has over 100,000 miles, and still does not smoke, and seems to make full power. Course this is now more than 10 years since I last opened it up.