Your bad luck with it wasn't because of car-part itself. It's just a site that searches salvage yards that upload their inventory to it. It's a convenient tool and you can search by distance if finding something more local is preferred. I've never tried uneedapart.com. I know I'd feel more comfortable putting my eyes and hands on something as important and expensive as a motor. But, after researching many yards, I've bought a few motors from a reputable yard in Virginia (M & M Auto Parts) and have gotten solid and honest deals from them.
As far as what motor, of course the easiest would be something just like what you're replacing- a Gen4 5.3 or 4.8. Personally, I'd avoid a 4.8 because I can feel the loss in torque. You can make it feel peppier by putting a taller gear in the rear end to compensate. A 6.0 would yield more power, but would also be a little thirstier. A good tune can make it as efficient as the 5.3 when cruising, though. I don't know what all the LMG came in, but a quick search of whatever motor you find can answer that. I'd guess that any 5.3 or 6.0 from '07 to '09 at least would be as close to a direct swap as possible. I think only the 2010+ motors had VVT in addition to AFM, and those are incompatible without swapping some parts.
