Each model of iPhone and iPod touch is identified by a unique model number. App developers choose what devices their app is designed for (obviously, for example, an app designed to use the iPhone's camera will be useless on an iPod touch), and so the app is restricted to only be used on the devices that the developer has selected. If you're trying to install an app on your iPod that is only designed for the iPhone, then you're out of luck - there is no way to "trick" iTunes or the App Store into thinking it's an iPhone because of the model number that is tied to the device.
So short answer, basically, it's not possible.