If you want to stick to the D300, I would do the 4:1. If you can't go fast enough, they are pretty smooth shifting into high range. Although, you may want to check out the strength rating of both sets of gears. With an auto, the gear reduction is nice, but not as necessary as it can be in a stick. Get the gears that are the strongest.
If the cash can happen, I agree with the others - an Atlas or Stak is always worth the investment! Staks run about the same price as an Atlas. Last i looked, 2 speed Staks were $2199, 3 speeds were $2799.