I suspect that you already know the answers and are just looking for opinions on this stuff, or you are a troll, trying to get people to start arguing for entertainment...
The 5.7/4l80e/4.1 (headers and a tune) in my '99 k2500 does pretty well while towing, but I have spent years tweaking this set up to make it work well. Especially with the extra wheels in the rear, you will have to get your set up perfect for this to be even an acceptable tow rig with either the 5.7 or 5.3. If you are willing to add boost to either of those, that is a different story, N/A, it will be low on power.
Or... Swap in a 7.4 with the typical tune/breathing mods and tow somewhat effortlessly. I say somewhat because people who haven't towed heavy stuff usually have totally unrealistic expectations of what the towing experience will be like. They then get upset and start throwing a ton of mods at their truck, which basically only make their wallets lighter. Towing with a gas motor (without boost) is never going to feel awesome. With boost it is only ok. Towing sucks.