I doubt fuel mileage will increase chief. Are you basing that just off of engine displacement? The 5.3 will have to spin higher to make the same power as that 6.0L, thus MORE fuel used because it will have to work harder to get the same job done. Id rebuild the 6.0L and be done. You'll have more "useable torque" where you need it for a truck with a larger displacement engine.
2004 A4 QSM GTO-Cold Air Inductions CAI -Hot Cam-Comp Magnum PRs and Comp trunion upgrade-otherwise stock-362hp/350tq
1967 Camaro--painted and rolling shell....for now