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Not much difference b/t the 4.8 & 5.3. Same bore, different stroke, 10hp more and about 25lbft torque (w/ 5.3). You'll notice a slightly faster pick-up from a dig with the 5.3, but is it really worth the swap hassle... that's for you to decide. 10hp/20lbft torque for $500 & 20hrs of labor. Worth your time or what?
Tuning for a 5.3 (stock-ish) is as easy as scaling the correct cyl volume, designating engine size, and airflow values for a smoother idle & driveability. Been there done that... ask me about a Yukon that blew a stock 5.3, and went to an auction with a perfectly running 4.8.
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20hrs?!?!?! Granted, I've never swapped a LSX engine but I've never had an engine swap take that long. Do these really take that long to change?