Not doubting you, but why would your Yukon with a "Z" engine not be a FF? If the 8th character of your VIN is "Z", you should have an RPO code (white sticker inside glovebox lid) that says "L59". For engine swap purposes, it's irrelevant if you never run Ethanol fuel, at least, nothing higher than E10. But, I had to mention it if you're looking at engine options. It'd be nice to have the FF intake manifold, or at least the injectors, since they can support more power, as with more cubes.
An LM7 would work fine. No power difference, though. As for the Summit one you linked to in your reply to Wes, I wouldn't go that route. It's not just about the numbers. Looking at the basic cam specs they listed, that engine could make those power figures, but they'd be all mid- to upper-end gains. Having a fullsize SUV, and the XL version at that, it'd be weaker accelerating from a stop and normal city cruising. It'd pull hard when flooring it to pass someone while already going highway speeds, though. But, how often are you high-speed racing your Yukon XL? You do a lot more accelerating from stops. Do any towing? You can do some things to help, such as a looser torque converter and lower rear end gearing. The engine already has 10:1 compression which helps power across the board. For about the same money, I'd get a rebuilt 6.0 with a well-designed cam. You can maintain and increase power across the whole RPM range and not sacrifice any in the usable/needed ranges. The key is to have the whole package work together- engine, cam, converter, trans and rear end gearing.
Also, I wouldn't call the 4L60 "trash". It does what it's designed to do just fine. Mine has just under 227K on it. The last 27K have been behind a ~400HP 5.3 including track time, lots of WOT, 2nd gear and 4,000 RPM throttle steers, 145+ MPH jaunts... It might be "trash" from a performance standpoint when compared to other options. But, it can be built to support pretty much anything one would reasonably do with a 3-ton land yacht.
What all do you wanna do with this rig? "More power"- sure. But are you looking for a particular number for the bragging rights? Do you wanna accelerate harder? Climb highway inclines more easily? Tow a trailer more easily? Race stock F-bodies? You can get more power to the wheels, which you'd feel, with just a good tune. Or do you wanna be able to roast the tires on a whim?