That trans, if I found the right data, is essentially a four speed with a double overdrive. 5th gear is .74/.80 and 6th is .60/.50, depending on the version. With that 6th gear ratio you can run a pretty steep diff gear and still have very reasonable rpm range at typical highway speeds. I would go with something in the 3.91 - 4.56 range. You may also need to consider the cam that comes in that engine and what rpm range it is making power. There are online calculators that you can put all your data in and figure out a good compromise. I wouldn't think that motor would be overly happy lugging down the highway at 1500 rpm which would be about where you'd be with a 3.08.