The 5.3L would be an excellent choice, as would the 4.2L Vortec I-6 from the new TrailBlazers and Envoys. The 4.2L is only available with an automatic, so I don't know how married you are to the idea of a manual trans. Also, if using a manual trans with the 5.3L, get a T-56 6-speed from a Camaro or Firebird. They may be a little more expensive than a T-5, but with two overdrives, your mileage would be excellent.