8 3/4 geared to low. Whats compatable?

1973 DDS 340, 727 trans, 8 3/4 (2070741) Here is my problem; My rear end gears seem to be real low. My tach is broke but I know its way to low. Even kids and people who know nothing of cars ask me why it doesnt shift. I new to Mopars and I dont know how easily rear end gears can be changed. I think the car was set-up not to drive on freeways. I dont think the previous owner used it daily. My son takes it to school every nite. Question#1. How hard is it to change out rear end gear? #2. Will a Duster 8 1/4 fit? Thnx. Oh yeah, does the 727 only shift 3 times?