I can't speak to the quality of the McGoughy's springs, but a friend local to me put the GF 2" kit on the rear of his truck, then chnaged his springs out to my old cut stockers to get a better ride quality.
Just saying.
As for how much to cut, start with less than you think (a full coil) and go from there. It's s fast and easy to raise the rear up, remove the springs and cut a little more, you can keep cutting 1/4 of a coil off until you get it where you want it.
PLUS, it will allow you to level the truck. Go measure your distances atall four corners from ground to fender lip. I bet neither the front nor rear are the same. Cutting will allow you to level that out too.