im sure the 500 barrel will go onto the 550 case,the sleeve lowers might look sloppy in the openings but the locating dowells should square it all up,the 550 top end wont go onto the 500 case unless those openings are machined slightly bigger i think,the 650 cam is a popular choice,unless you want to talk to a cam grinder and ask him,the performance section on here is the place for finding tricks for souping up your motor.
