I see alot of recomendations for Raptor Technique, so here is his info on header tanks. just my 2 bits

"Why do some say a header tank cures the problem of the engine getting too hot toward the end of the flight. It has to do with a couple of reasons, incorrect engine/throttle-pitch/mixing or vibration. The effects of either of these two will cause the engine to get hot. If it's due to vibration, then the header tank will help take the air bubbles out and therefore mask the true problem. If it's due to incorrect engine/throttle-pitch/mixing settings, then by having a header tank up high will give a rich run toward the end of the tank which would help cool the engine and therefore again mask the true problem."