Ignition point for gasoline is 495 degrees fahrenheit.
I'm guessing you only get those kinda of temps in the manifold and a little further down? (seeing as though my car does after fire every once in a while)
edit:
sounds good phantom... I'll put some kind of buffer between the tank and the exhaust maybe (bike innertube or something ghetto of the sort)
and I'm known in class for my incorrect use of my multifarious vocabulary
thanks guys