Actually.............
1. If properly designed, the parts of a V8 DO NOT see more stress than those of a properly designed 4. Stress is defined as force / area. Sure the forces are higher, but so is the area. Mechanical things are designed either to a maximum stress or a factor of safety.

2. If the 4-cylinder is more than 2 liters, chances are it has a pretty long stroke. Strokes of 3.7 to 3.9 are not uncommon in the 2.4 - 2.5 liter range.

3. Running the "big V8" at 3500 rpm for hours isn't outside what the designers built into these engines. The same basic engines were used for industrial purposes and many ran at 3000 or 3600 rpm for their entire existence. Trucks also run at that speed, at least they used to. If your engine comes apart after 100 hours of 3500rpm speeds you did something wrong.

4. No, the engine's natural frequency is not the same as the redline. The redline is the maximum safe speed for the engine based on the design stresses. I don't believe there is an identifiable natural frequency for the complete engine. There are natural frequencies for crankshaft, camshaft, rods, pushrods and valvesprings. There is a natural frequency for cylinders, and as they aren't all the same configuration, every cylinder could have its own. But as an assembly it would be very hard to identify a natural frequency for the engine.

5. There are two problems with running high rpm: wear and noise. Running the engine more rpm per mph (gear ratio) increases wear on the moving parts in the engine. This cannot be ignored. It is up to the owner to decide how much to tolerate. Higher driveshaft rpm with lower gears mean the driveshaft is closer to its natural frequency and stresses the universal joints more.
The same is true of noise. Noise increases with rpm, and noise causes fatigue in the driver. It doesn't matter so much to the passengers, they can take a nap. But the driver needs to stay alert.

I changed from 3.91s to 3.23s in my '64Dog and surprisingly saw no difference in gas mileage. But, after a 600 mile trip I was not so worn out with the 3.23s. I did notice a considerable dropoff in acceleration ability around town, and so that part of driving became less fun. I also did not want to invest thousands in an overdrive, although that would have solved both problems.

R.