Which ever way you install it, be sure the spring locating pin aligns with the hole in the spring mount. You will have to replace it if you mount it above the springs. Any trailer shop or spring shop will have new pins. Allen head bolts also work.
When above the springs, the axle tubes are likely to hit the frame rails.
To center the tires, you will have to drill a new locating hole in the spring pad or make a adapter with one pin and one hole offset. The u-bolts will still work as long as you don't move back to far. Most go about a 1" to 1.5" back, yours may vary.
Your choice of gear ratio is interesting, in overdrive, your final gear ratio will 1.91 :1 . A much better gear would be a 3.23, 3.55, or 3.73. Even a 3.08 will net a 2.16 to 1 ratio in overdrive. I really doubt your 300hp will be able to pull the overdrive with out eating a bunch of fuel. See here for a chart,
http://www.crawlpedia.com/rpm_gear_calculator.htmexample, 30" tall tire
2.73 gear ratio
60 mph
.70 over drive
1284 rpm, you better hope this is diesel with a ton of low rpm torque. Plus your burb is the size of a barn, so add in a bunch of wind resistance. Even my little 1/2 ton '37 truck eats fuel when the wind is strong, they are not aerodynamic in any way shape or form!
Joe
