with that size motor 2" headers would be good. they usually have 3 1/2" collectors. i put 3 1/2" pipes on mine and it was perfect with no more heat than any other size pipe. you don't really need a cross over pipe, they just let the motor idle a tad smoother. the bigger pipes will let the motor produce all the torque it can make. by the way that loop is the tits.
