Wheels have to come off. Wheel-well-guards can stay on. I also opted to drill a whole in the pcm bracket to gain access the top nut on the driver side motor mount although you will need a longer extention to get to it.
I just did this last weekend. I jacked up the motor until the axles touched the frame, then took the bottom two motor mount bracket bolts out and loosened the two top ones. Pretty easy to do with the wheels off and enough extensions. Much easier swap than jacking up the engine and grinding off the new bolts.With the wheel out of the way you'd gain more room to access the 3 bolts that hold the frame bracket for the motor mount. Added since you will notice that the new acdelco motor mount bolts will be longer than the orginal removing 1 or 2 will let you get them in there faster. It's way easier then trying to lift up the engine and busting something.