Diff ratio help

2.92 first gear, 0.8 5th gear, 0.63 6th gear. Rear tyres are 26 inches, haven't got dyno results yet and I don't want stupidly low revs on the highway - about 2000 - 2100 at 65mph is about it. With 650 horses, 5th gear is a power gear at 65mph if I want to overtake, but I love 4th for doing it in a quarter of the time. Sixrh gear is just a waste of a gear, unless you want to save on gas travelling freeways. I'm happy to use the gears, it's why I got it, so good cruising RPM at 65mph is what I'd like. Sounds like 3.7 to 3.9 would be much better suited to my needs? 3.9 more so?
You posted 315 40 18 size for the rear tires. Those are almost 29" tall. A 3.9 rear gear ratio will be a 2.45 final gear in OD. I don't think I'd do anything numerically lower than a 4.30.