what wheel and tire size can i use

Hey every one i'm new to this site.
I have a 74 Dodge dart swinger with a 318. I just got done swaping the the stock 7 1/4 rear end for an 8 3/4 rear end. The guy who owned the car befor me put a set off after market rims on it. Well now when one side of the rear end goes down and the other side goes up the tires rub the lip on the inside of the fender:banghead:. So I'm looking into some Mopar rally wheels.
What i want to know is can i put a 15 by 8 rim on the back and 15 by 7 up front or whould i have to put 15 by 7 all they way around. Then my next question is what size of tires would i have to put on the rims.
thanks for any and all help.