Stupid Questions.....again.

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So I'm thinking of buying a 5.3 off someone thats recommended by a friend, its got the heads off so anyone can check the cylinders and pistons, 84k on it, its intake to oil pan and only $500 but i think i could get it for $400. How hard would it be to use the 5.3 and drive on that while i build a 6.0 and then swap the two? What kind of issues would i run into there? I know id need a retune and small stuff but any big issues? And will a 4l80E fit a 5.3? I would think yes but I've heard multiple varying opinions from different people.