What is the advantage to7-4 and 5-3 cam swap

A couple of things here. The out going air charge of all Roots blowers exit the bottom of the blower at a very forward angle. This is the nature of the beast. The more twist in the rotors then the steeper the angle and with a high helix utilizing the forward pie opening it is a real high angle. Probable around 25 to 30 degrees. Like I said all the air is rushing forward at this steep angle and the has to turn 180 degrees and come back so you have a big distribution problem. Some motors I have seen have manifolds that block part of that pie opening and people just don't realize it so the have real bad distribution. Make sure your manifold matches the blower opening. The other thing is to becareful on balancing cylinders by using just Gets because that can really lead you astray and port jet sizing. I highly recommend that you only balance cylinders using sparjplug readings.