I've always heard 1.5 times the actual weight of the vehicle, or double just to be sure. I don't think they go by the GVWR. However, I say put the biggest one on there that will fit. The 1/2" plate you're mounting the winch to should eliminate any concerns of the weight and power of the winch. If the M12000 will fit, go with it. You may have to pull a stuck Super Duty or something heavier than your truck out of your way so you can go past or something. I think your biggest problem is going to be the height of the solenoid pack, but I think you can clock the winch over to where it will fit. In fact, I think Warn recommends that you mount the M12000 with the feet forward, if I'm not mistaken.
Are you going to put the fairlead in the gap between the valance and the bumper that's created by the body lift? A normal roller fairlead may be too tall and stick out too far, but you could always cut part of the valance out or remove it. Or you could go to synthetic rope and use a hawse fairlead. They stick out a lot less and I think they're a little shorter as far as height.
Man those bumpers are looking great. I'm impressed. I'm going to have to come up to N.C. and let you build me some!