Watch the Legato Stretches online guitar lesson by Jennifer Batten from Rock Sauce For Lead Guitar
Do an experiment where you put your left hand thumb above the fretboard and see how far you can stretch your fingers out. Then put the thumb in the middle of the neck and try the same stretch. You'll find you get a lot better stretch when the thumb is in the middle of the neck. Having the thumb up constricts your movement. I find it helpful to have the thumb up and gripping over the neck for certain bends and other moves, but for the most part it pretty much stays planted in the middle of the neck no matter where I'm playing on the board. That will help you with the stretches in these riffs.