I went from a 2000 Chevy 7.4 (454) to the 2005 Chevy 8.1 (496 cu) with the 5 speed Allison tranny. Night and day!!!! I would go with an 8.1 if you can find one, or the the Ford 6.8 V10. U-Haul used the 8.1 in the biggest trucks. The 8.1 is still being built, but it is used for marine purposes mostly.
2002 NuWa HitchHiker II 31 FKTG, HWH levelling, Onan Generator
2005 Chevy Silverado, Crew Cab, Long Bed, 8.1L/Allison
B&W Turnover Ball Hitch and Companion 5th Wheel Hitch