On a 396/325 in a 66 ss vert with factory ac, would a 7 blade 18 inch fan make a big difference over a 6 blade 18 inch?
Big Difference No! A difference yes. When you are running warm I have to ask WHEN are you running warm? At idle or cruising above 40 mph? If it is getting warm in stop and go traffic where you are idling most of the time look to your fan and shroud as not moving enough air through the radiator to cool the coolant in the radiator.
If you get warm while cruising down the road then your radiator is undersized for the heat load. The bigger or more powerful the engine the bigger the radiator has to be. Though it also has to be working properly. Is the radiator plugged up with scale? A look inside when it is cool will tell you the condition.
One third of the heat of combustion is shed by the radiator to the atmosphere (ambient or heat sink). One third goes to power the wheels and the final third is pumped out the exhaust as hot gasses.
This is just a concept as the actual numbers are closer to twenty percent of the heat drives the wheels and there is more heat lost out the exhaust, but I mention thirds to simplify the concept of Thermodynamics and Carnot efficiency. If you can cool the car's engine more and at a faster rate you can actually make more power without adding any other car parts than a bigger radiator.
A big semi-truck with it's over seven hundred cubic inch six cylinder engine makes about 350 horsepower, or about the same as a hot SBC. The difference is a semi truck doesn't have to look exotic or racy or different from last years model or different from another brand. It just has to work efficiently. That is why it has such a large radiator. It wants every scrap of power it can extract from burning the fuel. As such they cool it down to near room temperature to make as much power as possible.
Going back to thermodynamics. You are attempting to spread the bottom part of a graph as far as you can from the top of the graph to make as much power as you can. You can raise the top part by adding heat by way of higher compression (free horsepower the factory discovered in the sixties). Or you can add power through adding more fuel and air by way of a blower, turbocharger or nitrous oxide and more fuel. You can also make more power by cooling the engine down. It isn't your imagination that you notice your car is a lot more peppy on a cool autumn morning. It really is making more power as the engine is starting the bottom of the curve at a lower point so the difference which is the power it makes is greater.
In my car I installed a pick-up truck radiator because that radiator had the widest and tallest radiator I could find. I then ordered it extra thick to maximize the surface area the coolant had to shed heat to the atmosphere. I was able to keep a 582 cubic inch BBC making nearly a thousand horsepower cool in Florida's heat and humidity, even with A/C (have I mentioned how hot and humid it is in Florida). I also used a seven bladed mechanical fan with a full shroud that covered the entire radiator to force all the air through the radiator I included an air dam under the front end at the radiator core support. I had to modify the car to get the radiator to fit and still close the hood (this involved dropping the lower support down two inches and then welding the brackets back on as well as opening the core support more to expose all of the radiator).
From an engineering point of view it was worth it, but it won't win any judged car shows.