NEW Forum Group, SO-CAL Mopars

Alright so i am going to put a 340 or a 360 engine in my duster, what i am wondering is do i need to put a bigger rear end in? It has a 7 1/4 rear end in it i believe but i was told i could just put better gears in it and i would be alright. Is that true or will i just be better off putting in a bigger rear end in the back? and save me the trouble.