There is no difference in strength between the 3 and 4 series. If you think you might want to go with something like a 3.42 gear later then you should use the 3 series. You can get thick gears for the 3 series all the way to 4.88, so unles you think you might want to go with a 5.13 or something like that later I would recommend getting the 3 series. As for the Auburn, I don't care for them for several reasons that I won't go into right now. The Moroso is an Eaton with the 22 disc clutch set and 800 pound preload springs. They are not recommended for street use. The new Eaton has 400 pound preload springs and carbon fiber lining on the clutch plats. These are good for street use, but the clutches should probably be changed to the steel 18 disc set for drag racing.