Chevy Tow Vehicle Engine Advice

Now that the WakeWorld rig is in the graveyard, I have to replace it with another truck. It was a 2005 Chevy Silverado 3500 crew cab long bed 4x4 with the 8.1 engine and Allison transmission and I absolutely loved that truck! However, I don't tow as far as I used to, so I'm wondering if maybe I could get away with a smaller engine to save some gas mileage. I don't need the one-ton rating, so a 2500 will probably open up my selection a bit. I'm guessing I'll be looking at around the same model year because I'm really happy with that truck and I probably won't have enough money to get anything newer.

What kind of advice can you give me on the engines available in those trucks. I'm not a diesel fan, so you'll really have to do some convincing to get me to go that way. I'm just looking for gas engine advice. What engines are available (I think the 6.0 is the next biggest) and how do they tow? Thanks for your help!