Interesting. I don't believe I've ever seen a '71 with a 351W in it. Of course, that doesn't mean it isn't stock, just all the ones I've seen were Clevelands. Could it be a 302?
What are the casting numbers on the heads? Are the exhaust valves on the same "plane" as the intakes or "slanted" as compared? Is the thermostat in the intake or the block?
I would advise against the KB pistons (unless you're refering to the "Icon" forgings). They're known for ring gap issues and can cause failure if not exactly right. For similar or slightly more money, you can have forged pistons. Speed Pro are a bit heavy, but tough and "right" for the application. They use the stock ring pack, so good moly rings are relatively cheap.
After you answer the above questioins regarding the identity (not questioning your statement, just being thorough) of the engine, we can make more specific recommendations to get to a certain point.