V8 engines are odd fire motors. The exhaust pulses are not even. NA V8 engines gain power by utilizing exhaust scavenging. This is mainly done in 2 area's. With long tube headers the size, shape, length of the collectors can make or break an NA power band. Custom long tubes will be made to optimize the exhaust stroke and the cam. The size and length of the collector can actually be tuned move the torq curve of a power band. Shorty headers have no collector,.. thus no real scavenging if offered by using them. They only thing shorties would offer in the way of a performance gain would be by virtue of using larger tubes and less restrictive bends that cast manifolds create. But the full potential of any scavenging would be lost by using shorty headers.

The next area that would focus on scavenging would be in the mid pipe. A cross over in the mid pipe can open up as much as 5 - 10 hp. Running simply straight pipes with no cross over would be missing out on this.

With a basically stock motor like what your running, off the shelf parts will work best. If you run regular Flowmasters or any other chambered muffler,.. they will create all the back pressure the engine needs.