1973 340, want more power.

Well, a very discernable improvement in acceleration. Nothing comes without a price: with the higher rear gear ratio, the engine will rev more when cruising, will be noisier in the car (usually considered a bonus by the younger set LOL), and use more gas. Top end speed will be more limited. So choose wisely. At some point, you can get the rear gear so high as to just spin the tires too easily.

The SureGrip (Mopar-speak for a limited slip rear differential) will make it grip more surely.... a marketing term that really works! A nice upgrade for just about all things.