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I would think it has a lower gear, just compared to what I know is in mine. I have a 3.50:1 gear now, and I turn 2700 RPMs at 65 MPH.9" Rear end id, The 8" will hold up to a 351W mild build easy. If you are worried about the axles the 9" uses a 28 spline axle also. there is not many that have the big 31 splines in them.
your running 2800 rpms at 60 mph what gear are you thinking about going to. it looks like you are running a 3:00 rear gear now. The way you can get your rpms down more at 60mph is adding a over drive transmission.
With a factory 289, you would more than likely have the 8", and as MustangOne said, you should be fine with it unless you're making around 400hp.