the first question i have is how much power are you going to make from the 351? if you keep it stock then the stock six cylinder rear end will hold up, unless you are going to run slicks.
if you make less than 400hp on street tires, an 8" rear end will do nicely.
if you are going to run slicks, and/or make more than 400hp, then a 9 inch is the way to go.