Choosing a rear end
 

I own a 1971 Pontiac Lemans T37 and it has an open rear end on it. It has a built 350 running with a th350. i want to change out the rear end to a posi but dont know a whole lot about rear ends. I have 15" cragars all around with 275/60s in the back and 245/60s in the front. This is not a strip car but love racing it around town. What is a good ratio for me??? Thanks