Choosing a rearend

From Crankshaft Coalition Wiki

I just bought a 1982 monte carlo with a very strong 350 in it. the guy took great care of it. He said what it really needs to make the motor come alive is a good rear end. He mentioned a ss rear end he said is a 373. I DONT KNOW!! Help me out guys. what is a rear end that can give the power to this car it needs and still get decent gas mileage or is that to much to ask? Is it either power or performance or can you get alittle of both?