1. 1/2 watt resistors are way overkill. 1/4W or 1/8W are smaller, and the thinner leads are much easier to work with.
2. Bypassing the output resistor with a large capacitor increases the chance of an RF burn at the points of contact with the skin. This is particularly true if you use wrist bands instead of handles.
3. The combination of a bypass capacitor plus lowering the resistance of the output resistor increases the output energy *significantly*, and alters the frequency spectrum of the output signal. This forum is full of advice on timing, methods, and safety issues, but most of that goes out the window with such a large circuit change.
4. The output capacitor bypass doesn't need to be so large. For the 150 Ohm circuit, something around 1.0 uF to 2.2 uF will work about as well, and will probably be cheaper or easier to find. For the 480 Ohm/2128 Hz circuit, something around 0.1 uF to 0.22 uF will have about the same effect. Also, for any of these circuits, the bypass capacitor on pin 5 of the 555 can be just about anything. Usually, a 0.1 uF ceramic cap is cheaper and more available than a 0.01 uF film type.
5. Another way to save cost is to scale the resistor values so the timing capacitor at pin 6 and the output bypass capacitor are the same value and type. More on this if you want it.
6. This circuit does not include the latest modification to the original Clark circuit (an added resistor), but so what? I think that mod is counter-productive, although it makes a prettier picture on a scope. My vote - skip it.
7. With the lower value output resistor, added bypass capacitor, and 75% duty cycle, this circuit *is very different* from most "traditional" zapper designs. "Higher power" barely covers it. Be Careful.
8. If you want to start with something a bit less radical, try this. Start with the standard 1.0K output resistor instead of 150 Ohms. Start with no bypass cap to get a feel for the standard, then bypass the 1.0K resistor with a 0.022 uF or 0.033 uF film or ceramic capacitor. This will create a response pole about half way between the 727 Hz fundamental and the original Clark fundamental frequency of about 30 kHz. If that doesn't cause problems, you can try increasing the value of the bypass capacitor up to 0.22 uF or 0.33 uF. No warranties expressed or implied; I just think the circuit is safer with the larger value fixed resistor.