Like B stated block guards do more harm than good.
Picture it like this:
With boost pushing your motor, your motor is operating at higher temperatures. Therefore needing all the cooling it can get, block guards restrict the flow of coolant to the top of your motor.
400hp+ on a daily driven boosted SOHC is very very feasible if you did your homework and got your block sleeved. Most people want to go massive turbo and call it good. But contrary to what people may think, a big turbo may hurt your goals. To get the most out of your motor while boosting. You'll need to find a turbo that will spool at very low pressure and continue to spool throughout your RPM range. Thus keeping turbo "lag" in check and giving you a nice steady power climb.
Now there have been 400hp+ SOHC motors around on stock sleeves with block guards. Trust me I have seen one here in the state of Utah. Yeah, he did daily it but that did not last too long. Lasted maybe eight months tops if I remember right. When you increase your horsepower output of any motor your decreasing its reliability and longevity.
Last but not least TUNING is crucial especially for high horsepower motors!!!