There are a lot of considerations when it comes to engine size, side height, etc and a lot of it depends on the type of shooting that you do. I run a 20 x 7 1/2 (102" at the top rail) with 32" sides. 540 (builder quoted 650HP) pushing through a 84" 4 blade sensenrich prop and a 2.38 gear reduction. It will not run dry for long so you just have to be smart about it. I've filled the thing up with fish (literally) and was still able to run about 27MPH with 4 guys in the boat (was a bit of a struggle to get up on plane in deep water though). I have rounded chines and love the way it turns.
How length, side depth, motor size, etc all really depend on what you are hauling IMO. Guys down south that shoot dink gar can get away with an 18 foot boat and short sides. Up in here WI you would sink that damn thing on a good night. You need to match the motor size to the prop or you will just be blowing HP through the blades if they can't handle it. Not sure if I would explain this right, but the main reason I got a airboat was for shallow water fishing and hammering cattails. From idle to 2000 rpm it's more about torque IMO. A turbo would be nice, but im not sure if the time it takes for one to spool up would help on the low end when you are actually fishing in the shallows or not.
Good part about a 5.3 or a 6.0 are parts are readily available from about anywhere and you can get all kinds of cool diagnostic/tuner toys. You go to a parts store and ask for parts for a 540 and the kids behind the counter look at you like you are from another planet. Get a 6.0 and be happier. Everyone that has an airboat wants more engine, just like when they had fan boats. Plus, the added weight will help with keeping the back end down in the water.
